usb: musb: replace hard coded registers with defines

musb registers can be dumped using the file regdump
which is created in debugfs. Up to now  hard coded
register addresses are used for that. Different glue
layers however have different register addresses. The
patch addresses this issue by substituting bare register
addresses with defines.

Signed-off-by: Roman Byshko <rbyshko@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>