net/tap: do not set remote MAC if not necessary

net/tap: do not set remote MAC if not necessary

Check for the current MAC address on both the remote and the tap
netdevices before setting a new value.

While there, remove wrong empty lines and ensure tap_ioctl() return
value is negative, just like what is done throughout this code.

Fixes: 2bc06869cd94 ("net/tap: add remote netdevice traffic capture")

Signed-off-by: Pascal Mazon <pascal.mazon@6wind.com>