usb: musb: ensure in peripheral mode when checking session

The change ensures otg is not in a A- state when checking for VBUS in
peripheral mode.

musb_start() where VBUS checking is in can be called in many situations.
One example is in babble recovery routine, in which otg is transitioning
from A-HOST to A-WAIT-BCON, but VBUS discharge takes time, so
musb->is_active could be set to 1 due to this improper checking, then it
causes musb_bus_suspend() failed which leads to warning log message
flooding.

Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>