mmc: mxs: fix card detection function for broken card detect

Return -ENOSYS in get_cd if broken-cd is specified in the device tree.

Commit a91fe279

 (mmc: mxs: use standard flag for broken card
detection) sets MMC_CAP_NEEDS_POLL when broken-cd is specified. This
driver sets this flag unconditionally as it does not support a card
detect interrupt. Instead, broken-cd means that there is no card detect
signal connected.

The mmc core checks the get_cd function return value to determine if a
card is present. Only for a non-zero return value it will attempt to
initialize the card. So retuning -ENOSYS will allow the card to be
initialized.

For comparison, mmc_gpio_get_cd in slot-gpio.c also returns -ENOSYS if
the card detect GPIO is not valid.

Signed-off-by: Daniel Willmann <daniel@totalueberwachung.de>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <chris@printf.net>