ARM: dts: imx6qdl-gw5910: fix wlan regulator

Connect the wl_reg regulator to usdhc2 such that it can be enabled
and disabled as needed. There is no need for this to be always-on.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>