[snp] Retry initialisation if link is reported as down

[snp] Retry initialisation if link is reported as down

Some devices (observed with a Getac RX10 tablet and docking station
containing an embedded AX88179 USB NIC) seem to be capable of
detecting link state only during the call to Initialize(), and will
occasionally erroneously report that the link is down for the first
few such calls.

Work around these devices by retrying the Initialize() call multiple
times, terminating early if a link is detected.  The eventual absence
of a link is treated as a non-fatal error, since it is entirely
possible that the link really is down.

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