staging: fsl-mc: fix coding style warning

Fix for "WARNING: Avoid multiple line dereference - prefer
'mc_msi_domain->host_data'" found by checkpatch.pl in
bus/irq-gic-v3-its-fsl-mc-msi.c.

Signed-off-by: Lucian Zala <zala.lucian@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>