[PowerPC] Use < 0 and >= 0 for watchpoint ptrace calls

In commit 227c0bf4, which fixed some
watchpoint bugs, I compared the return value of some ptrace calls with ==
-1 and != -1.  Althought this should be correct, since the rest of the
file uses < 0 and >= 0, I have modified this for consistency.

gdb/ChangeLog:

	* ppc-linux-nat.c (ppc_linux_dreg_interface::detect)
	(ppc_linux_nat_target::low_prepare_to_resume): Use ptrace () < 0
	and >= to check return value instead of == -1 and != -1.