[iwm] iwm_{read,write}_prph() don't grab the nic lock in iwm themselves.

• Fix a couple of cases where the nic lock ended up not being grabbed during an iwm_read_prph() or iwm_write_prph().

Obtained from: dragonflybsd.git 6c5470f2db219c61e362c981fea969d97e1b8293