imx: serial: use tty_encode_baud_rate to set true rate

real baud rate may be different from the one requested.
for upper layers, set the nearest value to the real rate
in favour of the rate previously requested.

Signed-off-by: Oskar Schirmer <os@emlix.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>