net: dev_ioctl: remove redundant initialization of variable err

The variable err 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>
Link: https://lore.kernel.org/r/20201102121615.695196-1-colin.king@canonical.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>