libertas: fix CF firmware loading for some cards

if_cs_poll_while_fw_download() returned the number of iterations
remaining on success, which in turn got returned as the value from
if_cs_prog_real() and if_cs_prog_helper().  But since if_cs_probe()
interprets non-zero return values from firmware load functions as an
error, this sometimes caused spurious firmware load failures.

Signed-off-by: Dan Williams <dcbw@redhat.com>
Tested-by: Ryan Mallon <ryan@bluewatersys.com>
Acked-by: Ryan Mallon <ryan@bluewatersys.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>