mmc: dw_mmc: fix dw_mci_get_cd

bf626e55 ("mmc: dw_mmc: use slot-gpio to handle cd pin") caused
CDETECT to be ignored, since negated return value of mmc_gpio_get_cd(mmc)
can not be checked by IS_ERR_VALUE.

Also, add spin_lock_bh(&host->lock) for atomic access to
DW_MMC_CARD_PRESENT, otherwise sd detect may occasionally fail.

Signed-off-by: Zhangfei Gao <zhangfei.gao@linaro.org>
Reported-by: Kevin Hilman <khilman@linaro.org>
Reviewed-by: Sachin Kamat <sachin.kamat@linaro.org>
Tested-by: Sachin Kamat <sachin.kamat@linaro.org>
Tested-by: Kevin Hilman <khilman@linaro.org>
Acked-by: Seungwon Jeon <tgih.jun@samsung.com>
Signed-off-by: Chris Ball <chris@printf.net>