atm: idt77252: fix dev refcnt leak

init_card() calls dev_get_by_name() to get a network deceive. But it
doesn't decrease network device reference count after the device is
used.

Signed-off-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>