ravb: fix byte order for TX descriptor tag field lower bits

The wrong helper is used to swap the bytes when adding the lower bits of
the TX descriptors tag field in the shared ds_tagl variable. The
variable contains the DS[11:0] field and then the TAG[3:0] bits.

The mistake was highlighted by the sparse warning:

ravb_main.c:1622:31:    left side has type restricted __le16
ravb_main.c:1622:31:    right side has type unsigned short
ravb_main.c:1622:31: warning: invalid assignment: |=
ravb_main.c:1622:34: warning: cast to restricted __le16

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>