twrp: Fix unsigned compare compile warning.

Change-Id: I7e847e76932a6ceea3e669e8b8b9ec35e5ef9d9d
diff --git a/minuitwrp/graphics_fbdev.cpp b/minuitwrp/graphics_fbdev.cpp
index a1c42d0..8cf85f5 100644
--- a/minuitwrp/graphics_fbdev.cpp
+++ b/minuitwrp/graphics_fbdev.cpp
@@ -285,12 +285,10 @@
 static GRSurface* fbdev_flip(minui_backend* backend __unused) {
 #if defined(RECOVERY_BGRA)
     // In case of BGRA, do some byte swapping
-    unsigned int idx;
-    unsigned char tmp;
     unsigned char* ucfb_vaddr = (unsigned char*)gr_draw->data;
-    for (idx = 0 ; idx < (gr_draw->height * gr_draw->row_bytes);
+    for (int idx = 0 ; idx < (gr_draw->height * gr_draw->row_bytes);
             idx += 4) {
-        tmp = ucfb_vaddr[idx];
+        unsigned char tmp = ucfb_vaddr[idx];
         ucfb_vaddr[idx    ] = ucfb_vaddr[idx + 2];
         ucfb_vaddr[idx + 2] = tmp;
     }