tsnep: Subtract TSNEP_RX_INLINE_METADATA_SIZE once

Subtract size of metadata in front of received data only once. This
simplifies the RX code.

Signed-off-by: Gerhard Engleder <gerhard@engleder-embedded.com>
Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>