Driver core: Drop __must_check from bus_for_each_drv()

There is little rationale for marking bus_for_each_drv() __must_check.
It is more of an iteration helper than a real function. You don't know
in advance which callback it will be used on, so you have no clue how
important it can be to check the returned value. In practice, this
helper function can be used for best-effort tasks.

As a matter of fact, bus_for_each_dev() is not marked __must_check.
So remove it from bus_for_each_drv() as well. This is the same that
was done back in October 2006 by Russell King for
device_for_each_child(), for exactly the same reasons.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Andrew Morton <akpm@osdl.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>