arp: fix -H/-t handling.

While at it, shrank code.
function                                             old     new   delta
arp_main                                            1558    1487     -71

Signed-off-by: Kuleshov Aleksey <rndfax@yandex.ru>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
arp: code shrink

function                                             old     new   delta
packed_usage                                       29257   29252      -5
arp_main                                            1487    1471     -16

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
mdev: chdir back to /dev after trying to read firmware

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