OMAP5912: fix FIFO handling in UART driver

According to the OMAP5912 Serial Interfaces Reference Guide (see
http://focus.ti.com/lit/ug/spru760c/spru760c.pdf

, page 150), the
FIFO_EN enable bit in the FIFO Control Register (FCR) can only be
changed when the baud clock is not running, i. e. when both DLL and
DLH are set to 0.

Thus make sure that DLL and DLH are 0 when writing the FCR.

Signed-off-by: Wolfgang Denk <wd@denx.de>