igc: rename I225_RXPBSIZE_DEFAULT and I225_TXPBSIZE_DEFAULT

igc: rename I225_RXPBSIZE_DEFAULT and I225_TXPBSIZE_DEFAULT

Rename RX and TX packet buffer size macros in preparation for an
upcoming patch that will refactor buffer size handling using FIELD_PREP
and GENMASK.

Changes:
- Rename I225_RXPBSIZE_DEFAULT to IGC_RXPBSIZE_EXP_BMC_DEFAULT.
  The EXP_BMC suffix explicitly indicates Express and BMC buffer
  default values, improving readability and reusability for the
  upcoming changes, while also better reflecting the current buffer
  allocations.
- Rename I225_TXPBSIZE_DEFAULT to IGC_TXPBSIZE_DEFAULT.

These registers apply to both i225 and i226, so using the IGC prefix
aligns with existing macro naming conventions.

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>