Revert "microblaze_mmu_v2: Update signal returning address"

This reverts commit 8b28626a.

Offset -8 is wrong because when it is applied then one instruction
before brki r14, 8 is called again when we return.
Offset -4 is correct and brki instruction is called again.

This change came from ancient MMU kernel.

Signed-off-by: Michal Simek <monstr@monstr.eu>