drivers/pwm : remove critical section and mutex.

Use spinlock to replace critical section and mutex.

Signed-off-by: wangzhi16 <wangzhi16@xiaomi.com>

drivers/timers/pwm.c

@@ -316,10 +316,6 @@ static int pwm_start(FAR struct pwm_upperhalf_s *upper, unsigned int oflags)
 
   if (!upper->started)
     {
-      /* Disable interrupts to avoid race conditions */
-
-      flags = enter_critical_section();
-
       /* Indicate that if will be waiting for the pulse count to complete.
        * Note that we will only wait if a non-zero pulse count is specified
        * and if the PWM driver was opened in normal, blocking mode.  Also
@@ -369,8 +365,6 @@ static int pwm_start(FAR struct pwm_upperhalf_s *upper, unsigned int oflags)
           upper->started = false;
           upper->waiting = false;
         }
-
-      leave_critical_section(flags);
     }
 
   return ret;