Restore PBA setup for igb(4) class devices.

Do no write to PBA register on igb(4) devices unless we need
to make adjustments for the 82575 and jumbo frames.

Remove redundant LPE/~LPE assignments.

Move e1000_lv_jumbo_workaround_ich8lan() invokcation into a block
so that its not executed in the igb case.

Move em(4) class assignments of RCTL values to its own code block.

Adjust a few direct accesses of ifp->mtu to use accessor functions.

PR: 216734
Submitted by: Kaho Toshikazu <kaho@elam.kais.kyoto-u.ac.jp>