gpio: thunderx: fix error return code in thunderx_gpio_probe()

Fix to return error code -ENOMEM from the error handling
case instead of 0, as done elsewhere in this function.

Fixes: 5a2a3002

 ("gpio: Add gpio driver support for ThunderX and OCTEON-TX")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: David Daney <david.daney@cavium.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>