Running a tusb6010 system in host mode would fail with
testusb -a -t12 -c1 most of the time with BABBLE errors.

Turns out this was caused by incorrect handling of TX
FIFONOTEMPTY, where the code did not wait for it to clear.

This patch introduces musb_h_tx_flush_fifo(), and makes
musb_host.c to use it.
Signed-off-by: Tony Lindgren <tony@atomide.com>