mmc: core: Avoid negative index with array access

commit cf55a7ac upstream.

Commit 4d0c8d0a ("mmc: core: Use mrq.sbc in close-ended ffu") assigns
prev_idata = idatas[i - 1], but doesn't check that the iterator i is
greater than zero. Let's fix this by adding a check.

Fixes: 4d0c8d0a

 ("mmc: core: Use mrq.sbc in close-ended ffu")
Link: https://lore.kernel.org/all/20231129092535.3278-1-avri.altman@wdc.com/
Cc: stable@vger.kernel.org
Signed-off-by: Mikko Rapeli <mikko.rapeli@linaro.org>
Reviewed-by: Avri Altman <avri.altman@wdc.com>
Tested-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Link: https://lore.kernel.org/r/20240313133744.2405325-2-mikko.rapeli@linaro.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>