gpio: msm: Staticize local variable 'msm_gpio'

The local variable 'msm_gpio' is used only in this file.
Fix the following sparse warning:

drivers/gpio/gpio-msm-v2.c:109:21: warning: symbol 'msm_gpio' was not declared. Should it be static?

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>