mfd: mfd-core: Remove usage counting for .{en,dis}able() call-backs

The MFD implementation for reference counting was complex and unnecessary.
There was only one bona fide user which has now been converted to handle
the process in a different way. Any future resource protection, shared
enablement functions should be handed by the parent device, rather than
through the MFD subsystem API.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Mark Brown <broonie@kernel.org>