mmc: Fix MMC_CMD_STOP_TRANSMISSION response type and add comment

For MMC/eMMC, the MMC_CMD_STOP_TRANSMISSION response is R1 for read
transfers and R1b for write transfers per JEDEC Standard No. 84-B51
Page 126 . The response is R1b unconditionally per Physical Layer
Simplified Specification Version 9.00.

Correct the response type and add a comment about it.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Peng Fan <peng.fan@nxp.com>