serial: document start_rx member at struct uart_ops

Fix this doc build warning:
	./include/linux/serial_core.h:397: warning: Function parameter or member 'start_rx' not described in 'uart_ops'

Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Link: https://lore.kernel.org/r/5d07ae2eec8fbad87e623160f9926b178bef2744.1660829433.git.mchehab@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>