tty: mxser: make mxser_change_speed() return void

No other functions use the return value of mxser_change_speed() and the
return value is always 0 now. Make it return void. This fixes the
following coccicheck warning:

drivers/tty/mxser.c:645:5-8: Unneeded variable: "ret". Return "0" on
line 650

Signed-off-by: Jason Yan <yanaijie@huawei.com>
Link: https://lore.kernel.org/r/20200506061735.19369-1-yanaijie@huawei.com


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