fastboot: fix fastboot_set_reboot_flag()

In case CONFIG_FASTBOOT_FLASH_MMC_DEV == 0, compile-time condition
is not met and fastboot_set_reboot_flag() fails.

Fixes: a362ce21 ("fastboot: Implement generic fastboot_set_reboot_flag")
Signed-off-by: Roman Stratiienko <r.stratiienko@gmail.com>
Reviewed-by: Sean Anderson <seanga2@gmail.com>
Tested-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>