ARM: 6526/1: mmci: corrected calculation of clock div for ux500

The Ux500 variant of this block has a different divider.
The value used right now is too big and which means a loss
in performance. This fix corrects it. Also expand the math
comments a bit so it's clear what's happening. Further
the Ux500 variant does not like if we use the BYPASS bit,
instead we are supposed to set the clock divider to zero.

Signed-off-by: Ulf Hansson <ulf.hansson@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>