gpio: mt7621: report failure of devm_kasprintf()

kasprintf() may return NULL on failure of internal allocation thus the
assigned  label  is not safe if not explicitly checked. On error
mediatek_gpio_bank_probe() returns negative values so -ENOMEM in the
(unlikely) failure case should be fine here.

Fixes: 4ba9c3af

 ("gpio: mt7621: Add a driver for MT7621")
Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Reviewed-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Acked-by: Sean Wang <sean.wang@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>