mmc: mmc_spi: correct the while condition

When variable i will become 0, while(i--) loop breaks but variable i will
again decrement to -1 because of i-- and that's why below condition
"if (!i && (r != resp_match_value)" will never execute, So doing "i--"
inside of while() loop solves this problem.

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>