staging: mt7621-mmc: Fix null pointer deref if ext sdio irq enabled

The enable|disable_sdio_irq() fields of msdc_hw are never set, so when
they are called, they are null pointer. In case of the
MSDC_EXT_SDIO_IRQ flag set in msdc0_hw, this happens. But because
these fields are never set, they can simply be removed.

Signed-off-by: Christian Lütke-Stetzkamp <christian@lkamp.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>