ASoC: msm: 8226: Error handling for gpio failure

When driver queries for getting gpio number based
on the name, if gpio is not found on platform it
returns negative number.
Gpio number variable is unsigned int and driver will
treat error as a valid gpio pin.
This change modifies variable type to int.

Change-Id: Id889e6c0b04a06d5b517de392cd303cc6e1cd256
Signed-off-by: Gopikrishnaiah Anandan <agopik@codeaurora.org>