mtd: spi-nor: nxp-spifi: release flash_np in nxp_spifi_probe()

nxp_spifi_probe() increments refcnt of SPI flash device node by
of_get_next_available_child() and leaves it undecremented on both
successful and error paths.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>