greybus: power_supply: invalidate cache at update request

When we receive a update request we shall not trust the cache mechanism
and for that we need a way to invalidate the cache. Add a field that
will control the cache status and refactor the code to check if cache is
valid in a helper function.

This will fix the scenario where an update request is received within
the cache expiration time after a previous update as happened and would
be ignored.

Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>