usb: musb: allow musb and glue layers to be modules

This in part reverts commit 7a180e70cfc56e131bfe4796773df2acfc7d4180.
(usb: musb: temporarily make it bool) and while
at that we also allow glue layers to be compiled
as modules.

There are still some other changes needed
until we can have a fully functional build
with this setup, but we're getting there.

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