staging: fsl-dpaa2/eth: Cleanup TX frame freeing code

Cleanup code in free_tx_fd() that deals with S/G frames:
- remove local variables that aren't really needed
- in the frame sw annotation area, store the actual SG table
buffer size, which is needed on free, rather then recompute
it based on number of S/G entries

Signed-off-by: Ioana Radulescu <ruxandra.radulescu@nxp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>