mmc: tmio: remove unneeded variable in tmio_mmc_start_command()

Pass TMIO_MASK_CMD to tmio_mmc_enable_mmc_irqs() directly,
and remove the variable, irq_mask.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>