vfio/fsl-mc: Fix the dead code in vfio_fsl_mc_set_irq_trigger

Static analysis discovered that some code in vfio_fsl_mc_set_irq_trigger
is dead code. Fixed the code by changing the conditions order.

Fixes: cc0ee20b ("vfio/fsl-mc: trigger an interrupt via eventfd")
Reported-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Diana Craciun <diana.craciun@oss.nxp.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>