iwlagn: remove rxb page bookkeeping

We never use the value in alloc_rxb_page,
so there's no point in keeping it either.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>