igc: optimize TX packet buffer utilization for TSN mode

In preparation for upcoming frame preemption patches, optimize the TX
packet buffer size. The total packet buffer size (RX + TX) is 64KB, with
a maximum of 34KB for either RX or TX. Split the buffer evenly,
allocating 32KB to each.

For TX, assign 7KB to each of the four TX packet buffers (total 28KB)
and reserve 4KB for BMC.

References:
I225/I226 SW User Manual Section 4.7.9, Section 8.3.2

Co-developed-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.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>