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

(cherry picked from commit 12e8addd320df995bfb2b00f51c233541f741ae4)