mmc: sdhci: track whether preset mode is currently enabled in hardware

Track whether preset mode is currently enabled in hardware, and use that
when making decisions elsewhere in the code rather than reading the
register and checking the bit.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Tested-by: Markus Pargmann <mpa@pengutronix.de>
Tested-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <chris@printf.net>