[MMC] Indicate that R1/R1b contains command opcode

Some controllers actually check the first byte of the response (most
don't).  This byte contains the command opcode for R1/R1b and all 1:s
for other types. The difference must be indicated to the controller
so it knows which reply to expect.

Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>