usb: otg: mxs-phy: Fix mx23 operation

Currently mx23 fails to enumerate a USB device:

[ 1.300000] hub 1-0:1.0: unable to enumerate USB device on port 1
[ 1.520000] hub 1-0:1.0: unable to enumerate USB device on port 1
[ 1.740000] hub 1-0:1.0: unable to enumerate USB device on port 1
[ 1.960000] hub 1-0:1.0: unable to enumerate USB device on port 1
[ 2.180000] hub 1-0:1.0: unable to enumerate USB device on port 1

Use a kernel workqueue to asynchronously delay the setting of
ENHOSTDISCONDETECT bit until after higher level hub connect/reset processing
is complete.  Prematurely setting the bit prevents the connection
processing from completing and not setting it prevents disconnect from being
detected. No delay is needed for clearing of ENHOSTDISCONDETECT.

Successfully tested on mx23-olinuxino (micro, mini and maxi variants) and mx28evk.

Cc: stable@vger.kernel.org # v3.6
Signed-off-by: Mike Thompson <mpthompson@gmail.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>