driver/serial: remove return value of up_putc()

modify the prototype of up_putc(): remove the return value

The architecture code does not care about the return value of up_putc(), so removing it saves two statements:

Before:                                                    After:
de4c: e52de004  push  {lr}    @ (str lr, [sp, #-4]!)    |  de4c: e52de004  push  {lr}    @ (str lr, [sp, #-4]!)
de50: e24dd014  sub sp, sp, #20                         |  de50: e24dd014  sub sp, sp, #20
de54: e58d0004  str r0, [sp, #4]                        |  de54: e58d0004  str r0, [sp, #4]
de58: e30030f8  movw  r3, #248  @ 0xf8                  |  de58: e30030f8  movw  r3, #248  @ 0xf8
de5c: e3423000  movt  r3, #8192 @ 0x2000                |  de5c: e3423000  movt  r3, #8192 @ 0x2000
de60: e58d300c  str r3, [sp, #12]                       |  de60: e58d300c  str r3, [sp, #12]
de64: e59d1004  ldr r1, [sp, #4]                        |  de64: e59d1004  ldr r1, [sp, #4]
de68: e59d000c  ldr r0, [sp, #12]                       |  de68: e59d000c  ldr r0, [sp, #12]
de6c: ebfffe66  bl  d80c <pl011_putc>                   |  de6c: ebfffe66  bl  d80c <pl011_putc>
de70: e59d3004  ldr r3, [sp, #4]                        |  de70: e28dd014  add sp, sp, #20
de74: e1a00003  mov r0, r3                              |  de74: e49df004  pop {pc}    @ (ldr pc, [sp], #4)
de78: e28dd014  add sp, sp, #20                         |
de7c: e49df004  pop {pc}    @ (ldr pc, [sp], #4)        |

Signed-off-by: chao an <anchao@lixiang.com>

drivers/syslog/syslog_channel.c

@@ -235,10 +235,9 @@ static int syslog_default_putc(FAR syslog_channel_t *channel, int ch)
   UNUSED(channel);
 
 #  ifdef CONFIG_ARCH_LOWPUTC
-  return up_putc(ch);
-#  else
-  return ch;
+  up_putc(ch);
 #  endif
+  return ch;
 }
 
 static ssize_t syslog_default_write(FAR syslog_channel_t *channel,