regmap: Simplify implementation of the regmap_read_poll_timeout() macro

Simplify the implementation of the macro regmap_read_poll_timeout()
by using the macro read_poll_timeout().

Signed-off-by: Dejin Zheng <zhengdejin5@gmail.com>
Link: https://lore.kernel.org/r/20200420134647.9121-2-zhengdejin5@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>