mmc: change .get_ro() callback semantics

Now get_ro() callback must return 0/1 values for its logical states, and
negative errno values in case of error. If particular host instance doesn't
support RO/WP switch, it should return -ENOSYS.

This patch changes some hosts in two ways:

1. Now functions should be smart to not return negative values in
   "RO asserted" case (particularly gpio_ calls could return negative
   values for the outermost GPIOs).

   Also, board code usually passes get_ro() callbacks that directly return
   gpioreg & bit result, so at91_mci, imxmmc, pxamci and mmc_spi's get_ro()
   handlers need take special care when returning platform's values to the
   mmc core.

2. In case of host instance didn't implement get_ro() callback, it should
   really return -ENOSYS and let the mmc core decide what to do about it
   (mmc core thinks the same way as the hosts, so it isn't functional
   change).

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>

include/linux/mmc/host.h

@@ -58,6 +58,18 @@ struct mmc_host_ops {
 	 *
 	 * Also note that these functions might sleep, so don't call them
 	 * in the atomic contexts!
+	 *
+	 * Return values for the get_ro callback should be:
+	 *   0 for a read/write card
+	 *   1 for a read-only card
+	 *   -ENOSYS when not supported (equal to NULL callback)
+	 *   or a negative errno value when something bad happened
+	 *
+	 * Return values for the get_ro callback should be:
+	 *   0 for a absent card
+	 *   1 for a present card
+	 *   -ENOSYS when not supported (equal to NULL callback)
+	 *   or a negative errno value when something bad happened
 	 */
 	void	(*set_ios)(struct mmc_host *host, struct mmc_ios *ios);
 	int	(*get_ro)(struct mmc_host *host);