mmc: sdhci-esdhc-imx: remove emulation of uhs_mode

We no longer need to emulate the uhs_mode field of the host control2
register - the main sdhci driver never reads this back to evaluate
the current mode as it caches the current mode instead.

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>