[iwm] Add and use iwm_phy_db_free(), to plug phy_db memory leak.

(Together with other iwm(4) memory leak fixes) Memory leakage in M_DEVBUF
is now at ca. 2KB for each iwm(4) module load/unload cycle.

Submitted by: Imre Vadasz <imre@vdsz.com>
Approved by: re (gjb)
Obtained from: DragonflyBSD git eaf551a1d464c643e98ce5781971dd32124e9af1
Differential Revision: https://reviews.freebsd.org/D6819