sh: Fix ptrace hw_breakpoint handling

Since commit 34d0b5af50a063cded842716633501b38ff815fb it is no longer
possible to debug an application using singlestep. The old commit
converted singlestep handling via ptrace to hw_breakpoints. The
hw_breakpoint is disabled when an event is triggered and not re-enabled
again. This patch re-enables the existing hw_breakpoint before the
existing breakpoint is reused.

Signed-off-by: David Engraf <david.engraf@sysgo.com>
Cc: stable@kernel.org
Signed-off-by: Paul Mundt <lethal@linux-sh.org>