mmc: core: fix card detection regression

Since commit 89168b48 ("mmc: core: restore detect line inversion
semantics"), the SD card on i.MX28 (and possibly other) devices isn't
detected and booting stops at:

[    4.120617] Waiting for root device /dev/mmcblk0p3...

This is caused by the MMC_CAP2_CD_ACTIVE_HIGH flag being set incorrectly
when the host controller doesn't use a GPIO for card detection (but
instead uses a dedicated pin). In this case mmc_gpiod_request_cd() will
return before assigning to the gpio_invert variable, leaving the
variable uninitialized. The variable then gets used to set the flag.
This patch fixes the issue by making sure gpio_invert is set to false
when a GPIO isn't used. After this patch, i.MX28 boots fine.

The MMC_CAP2_RO_ACTIVE_HIGH (write protect) flag is also set incorrectly
for the exact same reason (it uses the same uninitialized variable), so
this patch fixes that too.

Fixes: 89168b48

 ("mmc: core: restore detect line inversion semantics")
Reported-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Kristina Martšenko <kristina.martsenko@gmail.com>
Tested-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>