greybus: interface: fix power_state documentation

Reword the power_state attribute documentation to make it clear that an
attempt to boot and possibly enumerate an interface is made when writing
"on" to the file, and that on errors the interface will again be powered
down.

Drop the incorrect claim that writing the currently read value has no
effect, since no such guarantees can be made (e.g. several writers may
be sleeping on the interface mutex).

Also fix some minor language issues.

Reviewed-by: Sandeep Patil <sspatil@google.com>
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Alex Elder <elder@linaro.org>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Patrick Titiano <ptitiano@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>