mmc: tmio_mmc: remove unused sdio_irq_enabled flag

The sdio_irq_enabled member of struct tmio_mmc_host is a left-over from the
previously removed SDIO IRQ workaround. It is no longer needed and can now
be removed too.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Chris Ball <cjb@laptop.org>