IXP4xx: Fix DMA masks.

Now, devices will have 32-bit default DMA masks (0xFFFFFFFF) as per DMA API.

Fixes:
$ ifconfig eth0 up
net eth0: coherent DMA mask is unset

$ ifconfig hdlc0 up
net hdlc0: coherent DMA mask is unset

Also fixes a cosmetic off-by-one bug which caused DMA transfers ending exactly
on the 64 MiB boundary to go through dmabounce unnecessarily.

Signed-off-by: Krzysztof Hałasa <khc@pm.waw.pl>
Tested-by: Simon Kagstrom <simon.kagstrom@netinsight.net>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>