ar71xx: correct button type for TL-MR3020 mode slider

ar71xx: correct button type for TL-MR3020 mode slider

The TP-Link TL-MR3020 has a three-state mode slider which was previously
integrated as a button (EV_KEY). This led to spurious activations of
failsafe mode.

Set the type for the button to switch (EV_SW), to avoid unintended
activations of failsafe mode.

Related: commit 27f3f493de06 ("gpio-button-hotplug: unify polled and
interrupt code")

Signed-off-by: David Bauer <mail@david-bauer.net>
(cherry picked from commit b017a016cc0cd26f84a7e6b8de3dc02dc101e888)