Add a function helper to factorize the hw_params code.

Suggested by Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: NBenoit Cousson <bcousson@baylibre.com>
Tested-by: NLars-Peter Clausen <lars@metafoo.de>
Reviewed-by: NLars-Peter Clausen <lars@metafoo.de>
Signed-off-by: NMark Brown <broonie@linaro.org>