As of commit 10d19bd ("[pxe] Always retrieve cached DHCPACK and apply
to relevant network device"), the UNDI driver has been the only user
of pxeparent_call().  Remove the unnecessary layer of abstraction by
refactoring this code back into undinet.c, and fix the ability of
undiisr.S to fall back to chaining to the original handler if we were
unable to unhook our own ISR.

This effectively reverts commit 337e1ed ("[pxe] Separate parent PXE
API caller from UNDINET driver").

Signed-off-by: Michael Brown <mcb30@ipxe.org>