loader: pxe.h constants have wrong values

FLTR_PRMSCS should be 0x4 and FLTR_SRC_RTG should be 0x8

PXE Specification 2.1 PXENV_UNDI_OPEN, page 59.

http://download.intel.com/design/archives/wfm/downloads/pxespec.pdf

Reviewed by: allanjude
Approved by: allanjude (mentor)
Differential Revision: https://reviews.freebsd.org/D10056