usb: musb: Fix use for of_property_read_bool for disabled multipoint

The value for the multipoint dts property is ignored when parsing with
of_property_read_bool, so we currently have multipoint always set as 1
even if value 0 is specified in the dts file.

Let's fix this to read the value too instead of just the property like
the binding documentation says as otherwise MUSB will fail to work
on devices with Mentor configuration that does not support multipoint.

Cc: Brian Hutchinson <b.hutchman@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>