ice: fix unused parameter warning

Depending on PAGE_SIZE, the following unused parameter warning can be
reported:

drivers/net/ethernet/intel/ice/ice_txrx.c: In function ‘ice_rx_frame_truesize’:
drivers/net/ethernet/intel/ice/ice_txrx.c:513:21: warning: unused parameter ‘size’ [-Wunused-parameter]
        unsigned int size)

The 'size' variable is used only when PAGE_SIZE >= 8192. Add __maybe_unused
to remove the warning.

Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>