usb: musb: temporarily make it bool

Due to the recent changes to musb's glue layers,
we can't compile musb-hdrc as a module - compilation
will break due to undefined symbol musb_debug. In
order to fix that, we need a big re-work of the
debug support on the MUSB driver.

Because that would mean a lot of new code coming
into the -rc series, it's best to defer that to
next merge window and for now just disable module
support for MUSB.

Once we get the refactor of the debugging support
done, we can simply revert this patch and things
will go back to normal again.

Cc: stable@kernel.org # v2.6.38
Signed-off-by: Felipe Balbi <balbi@ti.com>