mmc: sdhci-dove: use mmc_of_parse() and remove card_tasklet CD handler

f8ec589b ("mmc: sdhci-dove: allow GPIOs to be used for card detection
on Dove" added a gpio based card detect interrupt handler that was hooked
up into card_tasket.

3560db8e ("mmc: sdhci: push card_tasklet into threaded irq handler")
now removed that very card_tasklet causing sdhci-dove to fail on build
with:
drivers/mmc/host/sdhci-dove.c: In function 'sdhci_dove_carddetect_irq':
drivers/mmc/host/sdhci-dove.c:42:24: error: 'struct sdhci_host' has no member named 'card_tasklet'

To fix both the build error and get a working gpio card detection without
card_tasklet, replace sdhci_get_of_property() with more recent
mmc_of_parse(). It takes care of gpio-based card detect passed through DT
already and allows to remove the offending code sections dealing with
removed card_tasklet.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <chris@printf.net>