igc: field get conversion

Refactor the igc driver to use FIELD_GET() for mask and shift reads,
which reduces lines of code and adds clarity of intent.

This code was generated by the following coccinelle/spatch script and
then manually repaired in a later patch.

@get@
constant shift,mask;
type T;
expression a;
@@
-((T)((a) & mask) >> shift)
+FIELD_GET(mask, a)

and applied via:
spatch --sp-file field_prep.cocci --in-place --dir \
 drivers/net/ethernet/intel/

Cc: Julia Lawall <Julia.Lawall@inria.fr>
Reviewed-by: Marcin Szycik <marcin.szycik@linux.intel.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>