mmc: mmc_spi: Add Card Detect comments and fix CD GPIO case

mmc: mmc_spi: Add Card Detect comments and fix CD GPIO case

This patch fixes the MMC SPI driver from doing polling card detect when a
CD GPIO that supports interrupts is specified using the gpios DT property.

Without this patch the DT node below results in the following output:

 spi_gpio: spi-gpio { /* SD2 @ CN12 */
         compatible = "spi-gpio";
         #address-cells = <1>;
         #size-cells = <0>;
         gpio-sck = <&gpio6 16 GPIO_ACTIVE_HIGH>;
         gpio-mosi = <&gpio6 17 GPIO_ACTIVE_HIGH>;
         gpio-miso = <&gpio6 18 GPIO_ACTIVE_HIGH>;
         num-chipselects = <1>;
         cs-gpios = <&gpio6 21 GPIO_ACTIVE_LOW>;
         status = "okay";

         spi@0 {
                 compatible = "mmc-spi-slot";
                 reg = <0>;
                 voltage-ranges = <3200 3400>;
                 spi-max-frequency = <25000000>;
                 gpios = <&gpio6 22 GPIO_ACTIVE_LOW>;   /* CD */
         };
 };

 # dmesg | grep mmc
 mmc_spi spi32766.0: SD/MMC host mmc0, no WP, no poweroff, cd polling
 mmc0: host does not support reading read-only switch, assuming write-enable
 mmc0: new SDHC card on SPI
 mmcblk0: mmc0:0000 SU04G 3.69 GiB
 mmcblk0: p1

With this patch applied the "cd polling" portion above disappears.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Cc: stable@vger.kernel.org # v3.18+
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>