staging: most: i2c: prevent division by zero

This prevents division by zero scan_rate.

The zero scan_rate does not need any special action as it actually means
"never poll again".

Signed-off-by: Andrey Shvetsov <andrey.shvetsov@k2l.de>
Signed-off-by: Christian Gromm <christian.gromm@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>