The tran_speed field as decoded from csd is valid in most cases,
so use it to determine the maximum clock we can support. It is
not valid though for MMC highspeed cards, in this case the
csd contains invalid values and we have to set tran_speed explicitly
to the maximum speed. As the values passed into mci_set_clock are
now based on tran_speed we no longer have to limit the rate to
tran_speed in mci_set_clock.

The (intended) effect of this patch is that highspeed MMC cards are
no longer limited to non highspeed rates.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>