libertas: move firmware lifetime handling to firmware.c

Previously, each bus type was responsible for freeing the firmware
structure, but some did that badly.  Move responsibility for freeing
firmware into firmware.c so that it's done once and correctly, instead
of happening in multiple places in bus-specific code.

This fixes a use-after-free bug found by Dr. H. Nikolaus Schaller where
the SDIO code forgot to NULL priv->helper_fw after freeing it.

Signed-off-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>