net: macvtap: fix template string argument of device_create() call

The last argument of device_create() call should be a template string.
The tap_name variable should be the argument to the string, but not the
argument of the call itself.  We should add the template string and turn
tap_name into its argument.

Signed-off-by: Jean Sacren <sakiwit@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>