ppc4xx: Fix problem with extended program_tlb() funtion

The recently extended program_tlb() function had a problem when
multiple TLB's had to be setup (for example with 512MB of SDRAM). The
virtual address was not incremented. This patch fixes this issue
and is tested on Katmai with 512MB SDRAM.

Signed-off-by: Stefan Roese <sr@denx.de>