mmc: core: Add default timeout value for CMD6

EXT_CSD[248] includes the default maximum timeout for CMD6.
This field is added at eMMC4.5 Spec. And it can be used for default
timeout except for some operations which don't define the timeout
(i.e. background operation, sanitize, flush cache) in eMMC4.5 Spec.

Signed-off-by: Seungwon Jeon <tgih.jun@samsung.com>
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>