MLK-11790 dts: imx6: fix wlreg_on regulator always on issue

MLK-11790 dts: imx6: fix wlreg_on regulator always on issue

wlreg_on regulator always on will make WiFi module unable to do reset
during rmmod, then the re-insmod will fail.

In stead of make wlreg_on always on, we make vmmc regulator always on
for BT to work properly.
Then WiFi can still function well on wlreg_on regulator enable/disable.

Fixes: commit f17b5e5be (dts: mx6ul evk: set regulator wlreg_on always on)
Signed-off-by: Dong Aisheng <aisheng.dong@freescale.com>
(cherry picked from commit e92b727494b10c7a13e9491f55fc07a7ed7622a3)