tty: serial: define UART_LCR_WLEN() macro

Define a generic UART_LCR_WLEN() macro with a size argument. It can be
used to encode byte size into an LCR value. Therefore we can use it to
simplify the drivers using tty_get_char_size() in the next patches.
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220224095558.30929-1-jslaby@suse.czSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/serial.h

@@ -12,6 +12,8 @@
 #include <asm/page.h>
 #include <uapi/linux/serial.h>
 
+/* Helper for dealing with UART_LCR_WLEN* defines */
+#define UART_LCR_WLEN(x)	((x) - 5)
 
 /*
  * Counters of the input lines (CTS, DSR, RI, CD) interrupts