agp: remove unused variable arqsz in agp_3_5_enable()

This patch fix the following warning:
drivers/char/agp/isoch.c: In function ‘agp_3_5_enable’:
drivers/char/agp/isoch.c:322:13: warning: variable ‘arqsz’ set but not
used [-Wunused-but-set-variable]
  u32 isoch, arqsz;
             ^~~~~

Signed-off-by: Yunfeng Ye <yeyunfeng@huawei.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>