mmc: of_mmc_spi: fix unused warning

drivers/mmc/host/of_mmc_spi.c: In function 'mmc_spi_get_pdata':
drivers/mmc/host/of_mmc_spi.c:77:6: warning: variable 'ret' set but not used [-Wunused-but-set-variable]
  int ret = -EINVAL;
      ^

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>