pwm: pwm-qpnp: Fix the uninitialized local variable

"pwm_change_mode" returns the garbage value as the local variable
"rc" is returned without being initialized and updated. Fix it by
initializing the "rc" to "0".

Change-Id: Ifde4873fd91e13202c41fdf817bd85acc7ea36e0
Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>