mmc: block: make gen_err a bool variable

This gen_err flag is only assigned 0 or 1 and treated as a bool,
so convert it to a bool.

Cc: Chunyan Zhang <zhang.chunyan@linaro.org>
Cc: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>