ARM: 6767/1: ptrace: fix register indexing in GETHBPREGS request

The GETHBPREGS ptrace request incorrectly maps its index argument onto
the thread's saved debug state when the index != 0. This has not yet
been seen from userspace because GDB (the only user of this request)
only reads from register 0.

This patch fixes the indexing.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>