arping: avoid use of ether_ntoa(). Closes 8926

arping: avoid use of ether_ntoa(). Closes 8926

This is the only non-debug use of ether_ntoa(). By not using it,
we reduce bss:

function                                             old     new   delta
arping_main                                         1568    1665     +97
static.asc                                            18       -     -18
ether_ntoa                                            57       -     -57
------------------------------------------------------------------------------
(add/remove: 0/2 grow/shrink: 1/0 up/down: 97/-75)             Total: 22 bytes
   text    data     bss     dec     hex filename
 911020     493    7352  918865   e0551 busybox_old
 911069     493    7336  918898   e0572 busybox_unstripped

Also, "standard" arping zero-pads MAC. ether_ntoa() does not.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>