mmc: core: Allow setting CMD timeout for CMD6 (SWITCH).

CMD6 is an R1B-type command, where DAT is used as busy. Depending
on register written using CMD6, timeout value can be different
as per spec.

Signed-off-by: Andrei Warkentin <andreiw@motorola.com>
Signed-off-by: Chris Ball <cjb@laptop.org>