usb: musb: omap: write directly to mailbox instead of using phy

The glue layer should directly write to mailbox register (present in
control module) instead of calling phy layer to write to mailbox
register. Writing to mailbox register notifies the core of events like
device connect/disconnect.

Currently writing to control module register is taken care in this
driver which will be removed once the control module driver is in place.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>