powerpc/mpc85xx: Check return value of find_tlb_idx

find_tlb_idx() is called in board_early_init_r() on multiple boards.
The return value is not checked before being used to disable a TLB.
In normal case the return value wouldn't be -1. In case of a mis-
configuration during porting to a new board, checking the return value
may be helpful to reveal some user errors.

Signed-off-by: York Sun <yorksun@freescale.com>
diff --git a/board/freescale/mpc8572ds/mpc8572ds.c b/board/freescale/mpc8572ds/mpc8572ds.c
index 5686322..1bbf832 100644
--- a/board/freescale/mpc8572ds/mpc8572ds.c
+++ b/board/freescale/mpc8572ds/mpc8572ds.c
@@ -144,7 +144,7 @@
 int board_early_init_r(void)
 {
 	const unsigned int flashbase = CONFIG_SYS_FLASH_BASE;
-	const u8 flash_esel = find_tlb_idx((void *)flashbase, 1);
+	int flash_esel = find_tlb_idx((void *)flashbase, 1);
 
 	/*
 	 * Remap Boot flash + PROMJET region to caching-inhibited
@@ -155,8 +155,14 @@
 	flush_dcache();
 	invalidate_icache();
 
-	/* invalidate existing TLB entry for flash + promjet */
-	disable_tlb(flash_esel);
+	if (flash_esel == -1) {
+		/* very unlikely unless something is messed up */
+		puts("Error: Could not find TLB for FLASH BASE\n");
+		flash_esel = 2; /* give our best effort to continue */
+	} else {
+		/* invalidate existing TLB entry for flash + promjet */
+		disable_tlb(flash_esel);
+	}
 
 	set_tlb(1, flashbase, CONFIG_SYS_FLASH_BASE_PHYS,	/* tlb, epn, rpn */
 			MAS3_SX|MAS3_SW|MAS3_SR, MAS2_I|MAS2_G,	/* perms, wimge */