spl: mmc: Rename spl_boot_mode() to spl_mmc_boot_mode()

The function's name is misleading as one might think it is used
generally to select the boot-mode when in reality it is only used by the
MMC driver to find out in what way it should try reading U-Boot Proper
from a device (either using a filesystem, a raw sector/partition, or an
eMMC boot partition).

Rename it to spl_mmc_boot_mode() to make it more obvious what this
function is about.

Link: https://lists.denx.de/pipermail/u-boot/2020-April/405979.html


Signed-off-by: Harald Seiler <hws@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>