iwlwifi: pcie: support 2k rx buffers

The smallest rb size supported today is 4k rx buffers.
22560 devices use 2k rxb's, so allow using 2k buffers.

Signed-off-by: Golan Ben Ami <golan.ben.ami@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>