ethernet init hangs if no ethernet present

[wolfmanjm]

#0  0x140842fe in lpc43_ethreset (priv=0x10002624 <g_lpc43ethmac>) at chip/lpc43_ethernet.c:3423
#1  0x1408432c in lpc43_ifdown (dev=dev@entry=0x10002660 <g_lpc43ethmac+60>) at chip/lpc43_ethernet.c:2305
#2  0x14084a1a in lpc43_ethinitialize () at chip/lpc43_ethernet.c:3842
#3  up_netinitialize () at chip/lpc43_ethernet.c:3869
#4  0x1405c1e6 in up_initialize () at common/up_initialize.c:267
#5  0x14059e5a in os_start () at init/os_start.c:691
#6  0x1405bd4c in __start () at chip/lpc43_start.c:355

Certain boards will have no ethernet populated, and we do not want to have to support two different versions of nuttx, it would be nice if ethernet setup could gracefully fail if the ethernet chips are not present.