arch/arm/cpu/armv7/s5p-common/pwm.c: fix GCC 4.6 warning

Fix:
pwm.c: In function 'pwm_config':
pwm.c:85:16: warning: variable 'timer_rate_hz' set but not used
[-Wunused-but-set-variable]

Signed-off-by: Anatolij Gustschin <agust@denx.de>