[MMC] Improve MMC card block size selection

Select a block size for IO based on the read and write block size
combinations, and whether the card supports partial block reads
and/or partial block writes.

If we are able to satisfy block reads but not block writes, mark
the device read only.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

include/linux/mmc/card.h

@@ -30,7 +30,12 @@ struct mmc_csd {
 	unsigned int		tacc_ns;
 	unsigned int		max_dtr;
 	unsigned int		read_blkbits;
+	unsigned int		write_blkbits;
 	unsigned int		capacity;
+	unsigned int		read_partial:1,
+				read_misalign:1,
+				write_partial:1
+				write_misalign:1;
 };
 
 struct sd_scr {