msm: msm_watchdog: Don't pet the watchdog if it isn't enabled

If the kernel commandline has msm_watchdog.enable set to
zero and another driver calls pet_watchdog, the kernel will
panic since msm_tmr0_base is NULL (early return in probe).
Fix the pet_watchdog function to be a no-op if enable is
set to zero.

Change-Id: I4e1b72f9d98d5b4ca9ea312abde39dac7cb94b7d
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>