[pxeprefix] Work around missing type values from PXENV_UNDI_GET_NIC_TYPE

[pxeprefix] Work around missing type values from PXENV_UNDI_GET_NIC_TYPE

The implementation of PXENV_UNDI_GET_NIC_TYPE in some PXE ROMs
(observed with an Intel X710 ROM in a Dell PowerEdge R6515) will fail
to write the NicType byte, leaving it uninitialised.

Prepopulate the NicType byte with a highly unlikely value as a
sentinel to allow us to detect this, and assume that any such devices
are overwhelmingly likely to be PCI devices.

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