esp: Fix misplaced spin_unlock_bh.

A recent commit moved esp_alloc_tmp() out of a lock
protected region, but forgot to remove the unlock from
the error path. This patch removes the forgotten unlock.
While at it, remove some unneeded error assignments too.

Fixes: fca11ebd ("esp4: Reorganize esp_output")
Fixes: 383d0350 ("esp6: Reorganize esp_output")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>