mmc: mmc_spi: Convert 'multiple' to be boolean in mmc_spi_data_do()

Convert 'multiple' to be boolean in mmc_spi_data_do() since
it's initially being used as boolean.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20211206115218.73874-1-andriy.shevchenko@linux.intel.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>