igc: refactor TXDCTL macros to use FIELD_PREP and GEN_MASK

igc: refactor TXDCTL macros to use FIELD_PREP and GEN_MASK

Refactor TXDCTL macro handling to use FIELD_PREP and GENMASK macros.
This prepares the code for adding a new TXDCTL priority field in an
upcoming patch.

Verified that the macro values remain unchanged before and after
refactoring.

Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Faizal Rahim <faizal.abdul.rahim@linux.intel.com>
Tested-by: Mor Bar-Gabay <morx.bar.gabay@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>