regulator: Remove ifdefs for debugfs code

If CONFIG_DEBUG_FS=y debugfs functions will never return an
ERR_PTR. Instead they'll return NULL. The intent is to remove
ifdefs in calling code.

Update the code to reflect this. We gain an extra dentry pointer
per struct regulator and struct regulator_dev but that should be
ok because most distros have debugfs compiled in anyway.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>