net: atm: Remove redundant check.

Checking the 'adev' variable is unnecessary,
because 'cdev' has already been checked earlier.

Found by InfoTeCS on behalf of Linux Verification Center
(linuxtesting.org) with SVACE.

Fixes: 656d98b0 ("[ATM]: basic sysfs support for ATM devices")
Signed-off-by: Gavrilov Ilia <Ilia.Gavrilov@infotecs.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>