m25p80: Introduce an helper to retrieve the BlockBackend of a device

It will help in getting rid of some drive_get(IF_MTD) calls by
retrieving the BlockBackend directly from the m25p80 device.

Cc: Alistair Francis <alistair@alistair23.me>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Cédric Le Goater <clg@kaod.org>