ieee802154: hwsim: remove redundant initialization of variable res

The variable res is being initialized with a value that is
never read and it is being updated later with a new value.
The initialization is redundant and can be removed.

Addresses-Coverity: ("Unused value")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>