net: dsa: b53: remove redundant null check on dev

The pointer dev can never be null, the null check is redundant
and can be removed. Cleans up a static analysis warning that
pointer priv is dereferencing dev before dev is being null
checked.

Addresses-Coverity: ("Dereference before null check")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>