misra-c: Add 'U' to unsigned variable assignments in subsys/

Add 'U' to a value when assigning it to an unsigned variable.
MISRA-C rule 7.2

Signed-off-by: Patrik Flykt <patrik.flykt@intel.com>
diff --git a/subsys/fb/cfb.c b/subsys/fb/cfb.c
index dc2bc14..d0ef674 100644
--- a/subsys/fb/cfb.c
+++ b/subsys/fb/cfb.c
@@ -120,7 +120,7 @@
 	if ((fb->screen_info & SCREEN_INFO_MONO_VTILED) && !(y % 8)) {
 		for (size_t i = 0; i < strlen(str); i++) {
 			if (x + fptr->width > fb->x_res) {
-				x = 0;
+				x = 0U;
 				y += fptr->height;
 			}
 			x += fb->kerning + draw_char_vtmono(fb, str[i], x, y);
@@ -285,16 +285,16 @@
 
 	fb->x_res = cfg.x_resolution;
 	fb->y_res = cfg.y_resolution;
-	fb->ppt = 8;
+	fb->ppt = 8U;
 	fb->pixel_format = cfg.current_pixel_format;
 	fb->screen_info = cfg.screen_info;
 	fb->buf = NULL;
-	fb->font_idx = 0;
+	fb->font_idx = 0U;
 	fb->kerning = 0;
 	fb->inverted = false;
 
 	fb->fonts = __font_entry_start;
-	fb->font_idx = 0;
+	fb->font_idx = 0U;
 
 	fb->size = fb->x_res * fb->y_res / fb->ppt;
 	fb->buf = k_malloc(fb->size);