musb_hdrc runtime code shrinkage

Move musb_hdrc driver init into the init section, by switching over to use
platform_driver_probe().  Shrinks this driver's run-time footprint by about
5KB ... pretty good for linker level tweaks to just one (big) driver, and
there's a bit more shrinkage yet to be had (for DMA init).

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>