efi_loader: disk: allow blk devices even without UCLASS_PARTITION

While GPT partition is mandated in UEFI specification, CONFIG_PARTITION is
seen optional under the current implementation.
So modify efi_disk_rw_blocks() to allow accepting UCLASS_BLK devices.

Fixes: commit d97e98c8 ("efi_loader: disk: use udevice instead of blk_desc")
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Tested-by: Mark Kettenis <kettenis@openbsd.org>