Fix wait loop and void cast (#24)

* Simplify EINTR/ECANCEL error handling

1. Add semaphore uninterruptible wait function
2 .Replace semaphore wait loop with a single uninterruptible wait
3. Replace all sem_xxx to nxsem_xxx

* Unify the void cast usage

1. Remove void cast for function because many place ignore the returned value witout cast
2. Replace void cast for variable with UNUSED macro

drivers/bch/bchdev_driver.c

@@ -173,7 +173,7 @@ static int bch_close(FAR struct file *filep)
   /* Flush any dirty pages remaining in the cache */
 
   bchlib_semtake(bch);
-  (void)bchlib_flushsector(bch);
+  bchlib_flushsector(bch);
 
   /* Decrement the reference count (I don't use bchlib_decref() because I
    * want the entire close operation to be atomic wrt other driver

drivers/bch/bchdev_unregister.c

@@ -99,7 +99,7 @@ int bchdev_unregister(FAR const char *chardev)
 
   ret = file_ioctl(&filestruct, DIOC_GETPRIV,
                    (unsigned long)((uintptr_t)&bch));
-  (void)file_close(&filestruct);
+  file_close(&filestruct);
 
   if (ret < 0)
     {

drivers/bch/bchlib_cache.c

@@ -181,7 +181,7 @@ int bchlib_readsector(FAR struct bchlib_s *bch, size_t sector)
     {
       inode = bch->inode;
 
-      (void)bchlib_flushsector(bch);
+      bchlib_flushsector(bch);
       bch->sector = (size_t)-1;
 
       ret = inode->u.i_bops->read(inode, bch->buffer, sector, 1);

drivers/bch/bchlib_sem.c

@@ -57,19 +57,5 @@
 
 void bchlib_semtake(FAR struct bchlib_s *bch)
 {
-  int ret;
-
-  do
-    {
-      /* Take the semaphore (perhaps waiting) */
-
-      ret = nxsem_wait(&bch->sem);
-
-      /* The only case that an error should occur here is if the wait was
-       * awakened by a signal.
-       */
-
-      DEBUGASSERT(ret == OK || ret == -EINTR);
-    }
-  while (ret == -EINTR);
+  nxsem_wait_uninterruptible(&bch->sem);
 }

drivers/bch/bchlib_teardown.c

@@ -81,7 +81,7 @@ int bchlib_teardown(FAR void *handle)
 
   /* Close the block driver */
 
-  (void)close_blockdriver(bch->inode);
+  close_blockdriver(bch->inode);
 
   /* Free the BCH state structure */