enic: check return value for stat dump

We do not check the return value of enic_dev_stats_dump(). If allocation
fails, we will hit NULL pointer reference.

Return only if memory allocation fails. For other failures, we return the
previously recorded values.

Signed-off-by: Govindarajulu Varadarajan <_govind@gmx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>