83xx: mpc8315erdb: fix silly thinko in fdt_tsec1_fixup

The thinko was quite silly indeed, I messed with !ptr. Normally this
would trigger some fault, but in U-Boot NULL pointer is equal to phys
0, so the code was working still, just didn't actually test mpc8315erdb
environment variable value. Heh.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>