Remove usage of find_inferior in win32-i386-low.c

Remove usage of find_inferior in win32-i386-low.c

Straightforward replacement of find_inferior with the overload of
for_each_thread that filters on pid.  I am able to build-test this
patch, but not run it.

gdb/gdbserver/ChangeLog:

	* win32-i386-low.c (update_debug_registers_callback): Rename
	to ...
	(update_debug_registers): ... this, return void, remove pid_p arg.
	(x86_dr_low_set_addr, x86_dr_low_set_control): Use for_each_thread.