[MTD] Fix error checking after get_mtd_device() in get_sb_mtd functions
[MTD] Fix error checking after get_mtd_device() in get_sb_mtd functions

It returns ERR_PTR(foo) on error, not just NULL.

Signed-off-by: David Woodhouse <97b3379caa91f4ee97e44013ae4dc6350540fa9d@infradead.org>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs