iwlwifi: refactor rx register initialization

The patch adds HW bug W/A FH_RCSR_CHNL0_RX_IGNORE_RXF_EMPTY so that we
can enable again interrupt coalescing. It also uses named constants for
open code.

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