mtd: mxc-nand: don't disable clock in mxcnd-suspend

The clock must already be off after mtd->suspend.  Disabling it again
results in an negative overflow of the clock usage count.  This didn't
hurt as mxcnd_resume undid it after wake up.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>