tile/ptrace: Preserve previous registers for short regset write

Ensure that if userspace supplies insufficient data to
PTRACE_SETREGSET to fill all the registers, the thread's old
registers are preserved.

Cc: stable@vger.kernel.org
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Chris Metcalf <cmetcalf@mellanox.com>