MMC: Performance improvement for CONFIG_MMC_BLOCK_BOUNCE

MMC: Performance improvement for CONFIG_MMC_BLOCK_BOUNCE

This patch will increase performance significantly.It is supposed that
larger/bounce buffers will be used by mmc core if CONFIG_MMC_BLOCK_BOUNCE is
defined. But, in the mmc core, size of buffer is set to mmc->max_req_size if
mmc->max_req_size is smaller than MMC_QUEUE_BOUNCESZ i.e. bounce buffer size
(which is 64KB).By default, mmc->max_req_size is set to 4K. So, buffers of 4K
will be used instead of 64K even if CONFIG_MMC_BLOCK_BOUNCE is defined without
this patch. This patches forces mmc core to use bounce buffer of size
MMC_QUEUE_BOUNCESZ and so performance is increased.
In my test environment, write and Read speed without this patch was
2.6Mbytes/sec and 2.7Mbytes/sec respectively whereas write and read performance
with this patch has jumped to 4.4Mbytes/sec and 5MBytes/sec, respectively.
This testing has been done on SD Extreme III high speed 2GB SD card using
FAT file system with help of dd command

Signed-off-by: Purushotam Kumar <purushotam@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>