gpio: xra1403: Switch to a fixed upper bound for registers

Geert Uytterhoeven pointed out that the number of register was a
fixed upper bound so there's no need to use a dynamically allocated
array in place of a VLA. Use the defined upper bound.

Suggested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Laura Abbott <labbott@redhat.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>