The controller can't do anything else before it actually generates START/STOP.
So we check busy bit to make sure START/STOP is successfully finished.

If we don't check busy bit, START/STOP may fail on some fast CPUs.
Signed-off-by: NRichard Zhao <linuxzsc@gmail.com>
Signed-off-by: NBen Dooks <ben-linux@fluff.org>