usb: musb: drop unneeded musb_debug trickery

We have a generic way of enabling/disabling
different debug messages on a driver called
DYNAMIC_PRINTK. Anyone interested in enabling
just part of the debug messages, please read
the documentation under:

Documentation/dynamic-debug-howto.txt

for information on how to use that great
infrastructure.

Signed-off-by: Felipe Balbi <balbi@ti.com>