Blackfin: no-mpu: fix masking of small uncached dma region

When using an uncached DMA region less than 1 MiB, we try to mask off
the whole last 1 MiB for it.  Unfortunately, this fails as we forgot
to subtract one from the calculated mask, leading to the region still
be marked as cacheable.

Reported-by: Andrew Rook <andrew.rook@speakerbus.co.uk>
Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>