iwlwifi: accept up to 4K frame size on Rx side to fit A-MSDU frame

The driver drops any Rx frame larger than 2332 bytes, but with A-MSDU
frame, it could be up to 4K. This patch makes sure we can process larger
A-MSDU frame.

Signed-off-by: Mohamed Abbas <mabbas@linux.intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>