MMC: fix user capacity for partitioned eMMC card

if the card claims to be high capacity and the card
is partitioned the capacity shall still be read from
ext_csd SEC_COUNT even if the resulting capacity is
smaller than 2 GiB

Signed-off-by: Markus Niebel <Markus.Niebel@tq-group.com>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>