mmc: sd: fix the maximum au_size for SD3.0

Since SD Physical Layer specification V3.0, AU_SIZE is supported up
to 0xf.  So If SD-card is supported v3.0, then max_au should be 0xf.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>