adm80211: return an error if adm8211_alloc_rings() fails

We accidentally return success when adm8211_alloc_rings() fails but we
should preserve the error code.

Fixes: cc0b88cf ("[PATCH] Add adm8211 802.11b wireless driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>