usb: musb: dsps: check for the single bit

We want to check if that particular bit is
set. It could very well be that bootloader
(or romcode) has fiddled with MUSB before
us which could leave other bits set in this
register.

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