tsnep: Rework TX/RX queue initialization

Make initialization of TX and RX queues less dynamic by moving some
initialization from netdev open/close to device probing.

Additionally, move some initialization code to separate functions to
enable future use in other execution paths.

This is done as preparation for queue reconfigure at runtime, which is
necessary for XSK zero-copy support.

Signed-off-by: Gerhard Engleder <gerhard@engleder-embedded.com>
Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>