MIPS: Fix MAX_REG_OFFSET

Fix MAX_REG_OFFSET to point to the last register in 'pt_regs' and not to
the marker itself, which could allow regs_get_register() to return an
invalid offset.

Fixes: 40e084a5 ("MIPS: Add uprobes support.")
Suggested-by: Maciej W. Rozycki <macro@orcam.me.uk>
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>