mxssb: Fix sb_get_boot_list_size() type

The type must be unsigned int, otherwise gcc -Wextra produces a warning.
Signed-off-by: Marek Vasut <marex@denx.de>

mxssb.c

@@ -236,7 +236,7 @@ static struct sb_source_entry *sb_get_boot_list(const int cpu)
 	return 0;
 }
 
-static int sb_get_boot_list_size(const int cpu)
+static unsigned int sb_get_boot_list_size(const int cpu)
 {
 	switch (cpu) {
 	case CPU_MX23: