greybus: audio: don't inclide rwlock.h directly.

rwlock.h should not be included directly. Instead linux/splinlock.h
should be included. One thing it does is to break the RT build.

Cc: Vaibhav Agarwal <vaibhav.sr@gmail.com>
Cc: Johan Hovold <johan@kernel.org>
Cc: Alex Elder <elder@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: greybus-dev@lists.linaro.org
Cc: devel@driverdev.osuosl.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Mark Greer <mgreer@animalcreek.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>