gpio: mt7621: pass mediatek_gpio_bank_probe() failure up the stack

The error cases of mediatek_gpio_bank_probe() would go unnoticed (except
for the dev_err() messages). The probe function should return an error
if one of the banks failed to initialize properly indicated by
not returning non-0.

Fixes: 4ba9c3af

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