libertas: make lbs_ibss_join_existing() return error code on rates overflow

Commit e5e884b4 ("libertas: Fix two buffer overflows at parsing bss
descriptor") introduced a bounds check on the number of supplied rates to
lbs_ibss_join_existing() and made it to return on overflow.

However, the aforementioned commit doesn't set the return value accordingly
and thus, lbs_ibss_join_existing() would return with zero even though it
failed.

Make lbs_ibss_join_existing return -EINVAL in case the bounds check on the
number of supplied rates fails.

Fixes: e5e884b4

 ("libertas: Fix two buffer overflows at parsing bss descriptor")
Signed-off-by: Nicolai Stange <nstange@suse.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>