pinctrl: imx: fix unsigned check if nfuncs with less than or equal zero

The unsigned integer nfuncs is being error checked with a value less
or equal to zero; this is always false if of_get_child_count returns a
-ve for an error condition since nfuncs is not signed. Fix this by
making variables nfuncs and i signed integers.

Detected with Coccinelle:
drivers/pinctrl/freescale/pinctrl-imx.c:620:6-12: WARNING: Unsigned
expression compared with zero: nfuncs <= 0

Cc: Linus Walleij <linus.walleij@linaro.org>
Reported-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>