net/mlx5e: RX, Test the XDP program existence out of the handler

Instead of early return inside mlx5e_xdp_handle(), let the caller check
if an XDP program is loaded.  This allows saving a few unnecessary
function calls and calculations in case !prog.

Performance test: single core, drop packets in iptables
Before: 3,872,504 pps
After:  3,975,628 pps (+2.66%)

Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>