pinctrl: mediatek: fix check on EINT_NA comparison

Currently, the check on desc->eint.eint_n == EINT_NA is always false
because this is comparing a u16 to -1 which can never be true.  Fix
this by casting EINT_NA to u16.

Detected by CoverityScan, CID#1473610 ("Operands don't affect result")

Fixes: fb5fa8dc ("pinctrl: mediatek: extend struct mtk_pin_desc to pinctrl-mtk-common-v2.c")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>