mmc_spi: generate R1b response for erase and stop transmission command

As per the SD physical layer specification version 7.10, erase
command (CMD38) and stop transmission command (CMD12) will generate
R1b response.

R1b = R1 + busy signal

A non-zero value after the R1 response indicates card is ready for
next command.

Signed-off-by: Pragnesh Patel <pragnesh.patel@sifive.com>
Reviewed-by: Bin Meng <bin.meng@windriver.com>
Tested-by: Bin Meng <bin.meng@windriver.com>