libertas: return errno from lbs_add_card()

libertas: return errno from lbs_add_card()

This makes the error handling somewhat cleaner -- lbs_add_card() does no
logner throw away the errno and lets its callers propagate it.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>