iwlwifi: fix rx_chain computation

This patch fixes rx_chain computation. The code that adjusts number of
rx chains to number supported by HW was missing. Miss configuration
causes firmware error.  Note: iwlwifi supports HW with up to 3 RX
chains (2x2, 2x3, 1x2, and 3x3 MIMO). This patch also simplifies the
whole RX chain computation.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Mohamed Abbas <mohamed.abbas@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>