env: mmc: do not return an offset before the start of the partition

env: mmc: do not return an offset before the start of the partition

I have an GPT layout containing two partitions with the type GUID for
U-Boot environment:

        partition U-Boot-env-1 {
                offset = 0x1fc000
                size = 0x2000
                partition-type-uuid = "3de21764-95bd-54bd-a5c3-4abe786f38a8"
        }

        partition U-Boot-env-2 {
                offset = 0x1fe000
                size = 0x2000
                partition-type-uuid = "3de21764-95bd-54bd-a5c3-4abe786f38a8"
        }

and have set CONFIG_ENV_OFFSET=0x1fc000,
CONFIG_ENV_OFFSET_REDUND=0x1fe000 and CCONFIG_ENV_SIZE=0x2000.

This usually works just fine, but on an stm32mp, I was seeing weird
behaviour. It turns out that can be tracked down to that board setting
CONFIG_PARTITION_TYPE_GUID, so the logic in mmc.c ends up only finding
the first of the two partitions, but then in the copy=1 case ends up
computing 0x1fa000 as the *val returned (that is, the end of the
partition minus two times the environment size). That is of course
outside the found partition and leads to random corruption of the
partition preceding U-Boot-env-1.

Add a sanity check that the partition found is at least as large as
needed for the "one or two copies from the end of the partition" logic
to at least produce something within that partition. That will also
catch a bug where the partition is too small for even one copy of the
environment.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>