dfu: mmc: return data from raw partition reads

Nothing in dfu_mmc.c ever sets dfu->r_left. Consequently, reads from
MMC DFU altsettings would return 0 bytes.

Fix this by modifying dfu_read_medium_mmc() to set *len in the same way
that dfu_nand.c's dfu_read_medium_nand() does.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
---
Even though this mirrors how dfu_nand.c works, it feels odd to do this
every time a HW read is performed, since I believe this can happen
multiple times in a single DFU read transaction. Surely *len (and hence
dfu->r_left) should be set up once when the altsetting is initialized,
likely by calling a separate function in struct dfu_entity?
HACK: ARM: tegra: enable DFU too

FIXME: This should likely be in a separate tegra-common-dfu.h.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Merge remote-tracking branch 'u-boot-arm/master' into tegra_dev

Conflicts:
	boards.cfg
Merge remote-tracking branch 'u-boot-usb/pr-15052014' into tegra_dev
Merge branch 'tegra_dev__main' into tegra_dev
Merge branch 'tegra_dev__usb' into tegra_dev
Merge branch 'tegra_dev__arm' into tegra_dev