staging: wfx: fix error handling in wfx_tx_get_raw_link_id()

staging: wfx: fix error handling in wfx_tx_get_raw_link_id()

Since wfx_tx_get_raw_link_id() return an unsigned, it makes no sense to
return a negative value. "15" is a better value since it is used by
firmware for stations that have not yet associated link-ids.

Note that this should never happens since driver set max_ap_assoc_sta
to 14.

Fixes: 9bca45f3d692 ("staging: wfx: allow to send 802.11 frames")
Reported-by: kbuild test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20191011101639.31025-1-Jerome.Pouiller@silabs.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>