gpio: ep93xx: fix incorrect array element size check

Currently the while loop checks for the end of the array using
the size of egp->gc rather that the number of elements in the array,
so fix this. Also, perform the array size check first as stylistically
it is always good to bounds check on an array first before referencing
the array (in this case, we're just computing the address of an
element in an array so this is a moot point).

Fixes: fd935fc4 ("gpio: ep93xx: Do not pingpong irq numbers")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>