4xx_uart.c: fix GCC 4.6 build warnings

Fix:
4xx_uart.c: In function 'get_serial_clock':
4xx_uart.c:204:6: warning: variable 'tmp' set but not used
[-Wunused-but-set-variable]

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Stefan Roese <sr@denx.de>