batman-adv: rename gw_deselect() to gw_reselect()

The function batadv_gw_deselect() is actually not deselecting
anything. It is just informing the GW code to perform a
re-election procedure when possible.
The current gateway is not being touched at all and therefore
the name of this function is rather misleading.

Rename it to batadv_gw_reselect() to batadv_gw_reselect()
to make its behaviour easier to grasp.

Signed-off-by: Antonio Quartulli <antonio@open-mesh.com>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>