[MTD] [NAND] bf5xx_nand: Avoid crash if bfin_mac is installed.

http://blackfin.uclinux.org/gf/project/uclinux-dist/tracker/?action=TrackerItemEdit&tracker_item_id=4053



Singed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Bryan Wu <cooloney@kernel.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>