brcm47xx: resolve GPIO conflict for WRT54GSv1

brcm47xx: resolve GPIO conflict for WRT54GSv1

On the Linksys WRT54GSv1, the adm6996 switch driver and the
gpio_button_hotplug module both claim GPIO 6, which is connected to the
Reset button.  When the switch driver's request wins, the Reset button
cannot work. This makes it impossible to enter failsafe mode without a
serial console.

Stop requesting the "adm_rc" GPIO in the switch driver, since it is not
used anywhere.

Fixes FS#792.

Signed-off-by: Mirko Parthey <mirko.parthey@web.de>