mtd: rawnand: marvell: set reg_clk to NULL if it can't be obtained

Don't keep an error-pointer around in the private struct. If this optional
clock can't be obtained, simply set the pointer to NULL instead so we can
use clk_prepare_enable() on it without further checks,

Signed-off-by: Daniel Mack <daniel@zonque.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>