staging: greybus: audio_manager_sysfs: Replace sscanf with kstrto* to single variable conversion.

Fix checkpatch warning:

WARNING: Prefer kstrto<type> to single variable sscanf

kstrto* is designed to convert string to numerical value and makes
it easier to understand what the code does.

Signed-off-by: Elise Lennion <elise.lennion@gmail.com>
Reviewed-by: Mark Greer <mgreer@animalcreek.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>