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>