mtd: rawnand: vf610: Avoid a potential NULL pointer dereference

of_match_device can return NULL if there is no matching device. Avoid
a potential NULL pointer dereference by checking for the return value
and passing the error upstream.

Signed-off-by: Aditya Pakki <pakki001@umn.edu>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>