regulator: core: clean up debugfs monitoring and control features

Fix checkpatch warnings and other style issues in the regulator
debugfs functions.  Refactor the code to remove unnecessary error
checks as well as a shared buffer and mutex.  Rename variables
and functions to better follow the naming conventions found in
the core.c file.  Update the conditions used for allowing the
creation of 'voltage' and 'load' debugfs files.

Change-Id: I59881078ce9e908de11e74d15372edf233eb17b0
Signed-off-by: David Collins <collinsd@codeaurora.org>