This change removes the callback from atomic context which it doesn't
need to be in, and puts it in line with the debounced rescan.

This code is based on these e-mail threads with Christian Daudt:

  https://lkml.org/lkml/2013/8/19/539
  https://lkml.org/lkml/2014/3/19/79Signed-off-by: Markus Mayer <markus.mayer@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <chris@printf.net>