x25: remove redundant pointer dev

Pointer dev is being assigned a value that is never used, the assignment
and the variable are redundant and can be removed. Also replace null check
with the preferred !ptr idiom.

Cleans up clang scan warning:
net/x25/x25_proc.c:94:26: warning: Although the value stored to 'dev' is
used in the enclosing expression, the value is never actually read
from 'dev' [deadcode.DeadStores]

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Link: https://lore.kernel.org/r/20220508214500.60446-1-colin.i.king@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>