drivers: regulator: core: use pr_fmt

This adds a pr_fmt line which uses the __func__ macro. I also
convert the current pr_ lines to remove their __func__ usage.

Cc: bleong@codeaurora.org
Signed-off-by: Daniel Walker <dwalker@codeaurora.org>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>