ice: cleanup inconsistent code

It was found while doing further testing of the previous commit
fbf32a9bab91 ("ice: field get conversion") that one of the FIELD_GET
conversions should really be a FIELD_PREP. The previous code was styled
as a match to the FIELD_GET conversion, which always worked because the
shift value was 0.  The code makes way more sense as a FIELD_PREP and
was in fact the only FIELD_GET with two constant arguments in this
series.

Didn't squash this patch to make it easier to call out the
(non-impactful) bug.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Pucha Himasekhar Reddy <himasekharx.reddy.pucha@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>