mmc: sdhci: Convert sdhci_allocate_bounce_buffer() to return void

The function sdhci_allocate_bounce_buffer() always return zero at
present, so there's no need to have a return value, that will also make
error path easier.

CC: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Chunyan Zhang <zhang.chunyan@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>