hw/versatilepb, realview: Fix condition for instantiation of onboard NIC

Correct the condition determining whether we instantiate the onboard
NIC or a PCI card NIC on VersatilePB and Realview boards. This was broken
in two ways:
 (1) if the user asked for two default NICs ("-net nic -net nic") we would
crash trying to strcmp() a NULL pointer
 (2) if the user asked for two NICs explicitly of the same model as the
onboard NIC (eg "-net nic,model=smc91c111 -net nic,model=smc91c111")
we would try to instantiate two onboard NICs at the same address.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
