fix(scmi): change function prototype to fix gcc error

Change function prototype of plat_css_get_scmi_info() to fix the GCC
sign conversion error "comparison between signed and unsigned integer
expressions". Changing channel_id type to unsigned int since it can
never be a negative value.

Signed-off-by: Tony K Nadackal <tony.nadackal@arm.com>
Change-Id: I579b21497329db40897c10d86c8fc68e4877f3db
diff --git a/plat/arm/board/morello/morello_bl31_setup.c b/plat/arm/board/morello/morello_bl31_setup.c
index a044212..e04587d 100644
--- a/plat/arm/board/morello/morello_bl31_setup.c
+++ b/plat/arm/board/morello/morello_bl31_setup.c
@@ -19,7 +19,7 @@
 	.ring_doorbell = &mhu_ring_doorbell
 };
 
-scmi_channel_plat_info_t *plat_css_get_scmi_info(int channel_id)
+scmi_channel_plat_info_t *plat_css_get_scmi_info(unsigned int channel_id)
 {
 	return &morello_scmi_plat_info;
 }