if_rge: sync with up to date OpenBSD code

a0b4d9e5bd63984bc3ad25b7f2881741e0d73980
Add support for RTL8126 chip revision 0x64a00000 from Carl Henriksson.
While here, update and add microcode for the RTL8126 chipset variant.

46810ff569a9416642fc5f8e86501028008a2e95
For RTL8125, there's no need to check whether RGE_CMD_STOPREQ has been
completed. As with other chip variants, RGE_CMD_STOPREQ never acknowledges
completion.

5729fb17b5eeba56781da0b5c0525c198c5c2cd8
remove duplicate RGE_PHYSTAT_5000MBPS test

Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D54517
Obtained from: OpenBSD