agp: remove unused variable mcapndx

This patch fix the following warning:
drivers/char/agp/isoch.c: In function ‘agp_3_5_isochronous_node_enable’:
drivers/char/agp/isoch.c:87:5: warning: variable ‘mcapndx’ set but not
used [-Wunused-but-set-variable]
  u8 mcapndx;
     ^~~~~~~

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