mmc: emmc and hw partitions partition table id bugfix.

On bootup the emmc's hw partition is always set to 0 and the partition
table is read from it. When switching to another hw partition the
partition table's id is not updated but instead the old one from
hw partition 0 is used. If there is no partition table on hw partition 0
then the code will terminate and return error even if the desired hw
partition contains a perfectly fine partition table. This fix updates
the partition table struct to correspond to the specified hw partition
before testing if the partition table is valid or not.

Signed-off-by: Erik Tideman <erik.tideman@faltcom.se>
Reviewed-by: Tom Rini <trini@konsulko.com>
[trini: Squash the patch that corrected whitespace in the original into
this one, wrap with HAVE_BLOCK_DEVICE test]
Signed-off-by: Tom Rini <trini@konsulko.com>