iwlwifi: add IO function for continuous write of target memory

Add new IO function _iwl_write_targ_mem_words() to support
target memory write for a continuous area. It will return
error code -EBUSY if iwl_grab_nic_access() fails to indicate
the memory write does not be performed. Meanwhile the existing
function iwl_write_targ_mem() also been updated by using
_iwl_write_targ_mem_words() in a single word case.

Signed-off-by: Kenny Hsu <kenny.hsu@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>