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

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

Replace two usages with the overload of for_each_thread that filters on
pid.  It allows to simplify the callback a little bit.

gdb/gdbserver/ChangeLog:

	* linux-arm-low.c (struct update_registers_data): Remove.
	(update_registers_callback): Return void, take arguments
	directly, don't check thread's pid.
	(arm_insert_point, arm_remove_point): Use for_each_thread.