igc: Move igc_xdp_is_enabled()

Move the helper igc_xdp_is_enabled() to igc_xdp.h so it can be reused in
igc_xdp.c by upcoming patches that will introduce AF_XDP zero-copy
support to the driver.

Signed-off-by: Andre Guedes <andre.guedes@intel.com>
Signed-off-by: Vedang Patel <vedang.patel@intel.com>
Signed-off-by: Jithu Joseph <jithu.joseph@intel.com>
Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Tested-by: Dvora Fuxbrumer <dvorax.fuxbrumer@linux.intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>