tty: serial: Cleanup the bit shift with macro

This patch replaces the bit shift code with "_BITUL()" macro inside
"serial_rs485" struct.

Signed-off-by: Crescent CY Hsieh <crescentcy.hsieh@moxa.com>
Link: https://lore.kernel.org/r/20231201071554.258607-2-crescentcy.hsieh@moxa.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>