Blackfin: bfin_mac: drop volatile markings on packet buffers

Now that common code doesn't declare these as volatile, we don't need to
either anymore.  This fixes the build warning:

bfin_mac.c: In function 'bfin_EMAC_recv':
bfin_mac.c:193:23: warning: assignment discards qualifiers from pointer target type

Signed-off-by: Mike Frysinger <vapier@gentoo.org>