iwn: reduce code duplication in iwn_read_firmware()

• Separate 'firmware_put(sc->fw_fp, FIRMWARE_UNLOAD); sc->fw_fp = NULL;'

into iwn_unload_firmware().

• Move error handling to the end of iwn_read_firmware().

No functional changes.

Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D4768