igc: use FIELD_PREP and GENMASK for existing RX packet buffer size

Prepare for an upcoming patch that modifies the RX buffer size in TSN mode.
Refactor IGC_RXPBSIZE_EXP_BMC_DEFAULT and IGC_RXPBS_CFG_TS_EN using
FIELD_PREP and GENMASK to improve clarity and maintainability. Refactor
both macros for consistency, even though the upcoming patch only use
IGC_RXPBSIZE_EXP_BMC_DEFAULT.

The newly introduced macros follow the naming from the i226 SW User Manual
for easy reference.

I've tested IGC_RXPBSIZE_EXP_BMC_DEFAULT and IGC_RXPBS_CFG_TS_EN before
and after the refactoring, and their values remain unchanged.

Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
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>