staging: most: net: make net device lifetime obvious

The function aim_probe_channel calls only one of the functions
alloc_netdev and register_netdev per run.

Correspondingly, the function aim_disconnect_channel calls only one of
the functions unregister_netdev and free_netdev per run.

This patch makes it obvious by using the 'else' part of the 'if'
statement.

Signed-off-by: Andrey Shvetsov <andrey.shvetsov@k2l.de>
Signed-off-by: Christian Gromm <christian.gromm@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>