net, ppp: Report correct error code if unit allocation failed

Allocating unit from ird might return several error codes
not only -EAGAIN, so it should not be changed and returned
precisely. Same time unit release procedure should be invoked
only if device is unregistering.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
CC: Paul Mackerras <paulus@samba.org>
Signed-off-by: David S. Miller <davem@davemloft.net>