mtd: mxc-nand: no need to check for validity of platform driver data

The probe function calls platform_set_drvdata with a valid pointer when
the probe is successful.  As mxcnd_suspend and mxcnd_resume are only
called on bound devices, platform_get_drvdata always returns non-NULL.

This fix isn't critical as the pointer is always valid so it doesn't
matter if the compiler generated code for it or not.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reported-by: David Binderman <dcb314@hotmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>