Currently RTT mmc stack only support Highspeed mode or
blow, which means the max speed should be 52MHz according
to JEDEC spec. Two problems show here:

(1) max_data_rate = (unsigned int)-1. The value of unsigned int
depends on compilers/arch. Moreover, it makes no sense to assume
cpu addressing width with IP clock rate limit.
(1)hs_max_data_rate was set to 200MHz.

So what should BSP drivers do if 52MHz < max_data_rate < 200MHz?
Either it blindly sets a spec-violated clock rate to drive a Highspeed
card, or just adjust the clock rate internally. Both cases are
really bad for practice.

If the card claims to support Highspeed, we set the clock to not
to exceed 52MHz. Otherwise it should be set according to
card->max_data_rate parsed by ext_csd. This patch fixes it as-is,
and also simplify the code a lot.
Signed-off-by: NShawn Lin <shawn.lin@rock-chips.com>