powerpc/booke: Fix breakpoint/watchpoint one-shot behavior

Another fix for the extended ptrace patches in the -next tree.

The handling of breakpoints and watchpoints is inconsistent.  When a
breakpoint or watchpoint is hit, the interrupt handler is clearing the
proper bits in the dbcr* registers, but leaving the dac* and iac* registers
alone.  The ptrace code to delete the break/watchpoints checks the dac* and
iac* registers for zero to determine if they are enabled.  Instead, they
should check the dbcr* bits.

Signed-off-by: Dave Kleikamp <shaggy@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>