staging: fsl-mc: make function its_fsl_mc_msi_init static

The function its_fsl_mc_msi_init is local to the source and does
not need to be in global scope, so make it static.

Cleans up sparse warning:
symbol 'its_fsl_mc_msi_init' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>