net: mvneta: avoid unnecessary xdp_buff initialization

Explicitly initialize mandatory fields defining xdp_buff struct
in mvneta_rx_swbm routine

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Reviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>