musb_hdrc: Clean-up TUSB fifo access

This patch cleans-up TUSB fifo access and uses readsl/writesl()
when possible. It also has hooks for some further optimization,
notably synchronous burst access using PIO.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>