[AGPGART] alpha-agp warning fix

drivers/char/agp/alpha-agp.c:138: warning: initialization from incompatible pointer type
drivers/char/agp/alpha-agp.c:139: warning: initialization from incompatible pointer type

Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Richard Henderson <rth@twiddle.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Dave Jones <davej@redhat.com>