drivers/serial: Always support c_oflag, c_iflag and c_lflag in termios

CONFIG_SERIAL_TERMIOS only decide whether to support c_cflag field since
many terminal application need the first three fields to work correctly.
For more information please reference:
https://www.mail-archive.com/dev@nuttx.apache.org/msg09321.html

before this change(olimexino-stm32:tiny):
   text    data     bss     dec     hex filename
  34884     328    1768   36980    9074 nuttx
after this change:
   text    data     bss     dec     hex filename
  35052     340    1768   37160    9128 nuttx
delta
   text    data     bss     dec     hex filename
    168      12       0     180      b4 nuttx

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>

boards/z80/ez80/z20x/src/w25_main.c

@@ -545,7 +545,6 @@ static int w25_boot_program(void)
       return ret;
     }
 
-#ifdef CONFIG_SERIAL_TERMIOS
   /* Drain all pending Tx output in stdout. "Booting..." message will be
    * lost if the outgoing Tx bytes are not drained.
    */
@@ -557,7 +556,6 @@ static int w25_boot_program(void)
       fprintf(stderr, "ERROR: tcdrain() failed: %d\n", ret);
       return ret;
     }
-#endif
 
   /* Start the successfully loaded program */