tsnep: Fix tsnep_tx_unmap() error path usage

If tsnep_tx_map() fails, then tsnep_tx_unmap() shall start at the write
index like tsnep_tx_map(). This is different to the normal operation.
Thus, add an additional parameter to tsnep_tx_unmap() to enable start at
different positions for successful TX and failed TX.

Fixes: 403f69bb

 ("tsnep: Add TSN endpoint Ethernet MAC driver")
Signed-off-by: Gerhard Engleder <gerhard@engleder-embedded.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>