ubi: fix swapped arguments to call to ubi_alloc_aeb

Static analysis by CoverityScan detected the ec and pnum
arguments are in the wrong order on a call to ubi_alloc_aeb.
Swap the order to fix this.

Fixes: 91f4285f ("UBI: provide helpers to allocate and free aeb elements")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Richard Weinberger <richard@nod.at>