iwlwifi: Add iwl_write8()

To support byte writes to CSR_INT_COALESCING and CSR_INT_PERIODIC registers,
add iwl_write8(), including debug/trace support.

Signed-off-by: Ben Cahill <ben.m.cahill@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>