iwlagn: fix stack corruption

Alexander reported a strange crash in iwlagn that
Meenakshi and Wey couldn't reproduce. I just ran
into the same issue and tracked it down to stack
corruption. This fixes it.

The problem was introduced in
commit 4b8b99b6e650d0527f3a123744b7459976581d14
Author: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Date:   Fri Jul 8 14:29:48 2011 -0700

    iwlagn: radio sensor offset in le16 format

Cc: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Cc: Meenakshi Venkataraman <meenakshi.venkataraman@intel.com>
Reported-by: Alexander Diewald <alex@diewald.cc>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>