pinctrl: pinctrl-mxs: Fix variables' definition type

Fix the following warnings when building with W=1 option:

drivers/pinctrl/pinctrl-mxs.c: In function 'mxs_dt_free_map':
drivers/pinctrl/pinctrl-mxs.c:151:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
drivers/pinctrl/pinctrl-mxs.c: In function 'mxs_pinctrl_enable':
drivers/pinctrl/pinctrl-mxs.c:208:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
drivers/pinctrl/pinctrl-mxs.c: In function 'mxs_pinconf_group_set':
drivers/pinctrl/pinctrl-mxs.c:265:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
drivers/pinctrl/pinctrl-mxs.c: In function 'mxs_pinctrl_parse_group':
drivers/pinctrl/pinctrl-mxs.c:376:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>