serial: opencores_yanu: Avoid duplicate oc_serial_setbrg() implementation

The implementation of oc_serial_setbrg() for CONFIG_SYS_NIOS_FIXEDBAUD and
!CONFIG_SYS_NIOS_FIXEDBAUD are very similar.
Add a baudrate variable and set it to either CONFIG_BAUDRATE or gd->baudrate.
Then we can unify the code for both cases.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Thomas Chou <thomas@wytron.com.tw>