usb: musb: dsps: keep VBUS on for host-only mode

Currently VBUS is turned off while a usb device is detached, and turned
on again by the polling routine. This short period VBUS loss prevents
usb modem to switch mode.

VBUS should be constantly on for host-only mode, so this changes the
driver to not turn off VBUS for host-only mode.

Fixes: 2f3fd2c5 ("usb: musb: Prepare dsps glue layer for PM runtime support")
Cc: stable@vger.kernel.org #v4.11
Reported-by: Moreno Bartalucci <moreno.bartalucci@tecnorama.it>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>