net: macb: remove extraneous return when MACB_EXT_DESC is defined

When macro MACB_EXT_DESC is defined we end up with two identical
return statements and just one is sufficient. Remove the extra
return.

Detected by CoverityScan, CID#1449361 ("Structurally dead code")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>