Remove usages of find_inferior in linux-mips-low.c

Remove usages of find_inferior in linux-mips-low.c

Replace with for_each_thread with pid filtering.  This allows
simplifying the callback a little bit.

gdb/gdbserver/ChangeLog:

	* linux-mips-low.c (update_watch_registers_callback): Return
	void, remove pid_p parameter, don't check for pid.
	(mips_insert_point, mips_remove_point): Use for_each_thread.