serial: max310x: Setup baud rate generator more precisely

This patch provide more precisely setup of baud rate generator.
If the result of division has a remainder, we use the multiplier
for the base frequency. Additionally, we report result baud rate
back to serial core.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>