regulator: qpnp-amoled: Fix uninitialized variable warning

Variable 'rc' is used without being initialized. Fix it by
initializing it. While at it, return 'rc' instead of '0' at
the end of the function.

Change-Id: I33e33a4fff9c9d63d2c20de5971fbb417e01433c
Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>