ARM: 6657/1: hw_breakpoint: fix ptrace breakpoint advertising on unsupported arch

The ptrace debug information register was advertising breakpoint and
watchpoint resources for unsupported debug architectures. This meant
that setting breakpoints on these architectures would appear to succeed,
although they would never fire in reality.

This patch fixes the breakpoint slot probing so that it returns 0 when
running on an unsupported debug architecture.

Reported-by: Ulrich Weigand <ulrich.weigand@de.ibm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>