[Blackfin] arch: fix building with mtd uclinux by putting the mtd_phys option...

[Blackfin] arch: fix building with mtd uclinux by putting the mtd_phys option into the function it actually gets used in

Signed-off-by: Mike Frysinger <vapier.adi@gmail.com>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>