e1000: rctl/srrctl buffer size init, rfctl fix
Simplify the setup of srrctl.BSIZEPKT on igb class NICs.
Improve the setup of rctl.BSIZE on lem and em class NICs.
Don't try to touch rfctl on lem class NICs.
Manipulate rctl.BSEX correctly on lem and em class NICs.
Approved by: markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D31457