simplify pnp_activate_dev() and pnp_disable_dev() return values

Make pnp_activate_dev() and pnp_disable_dev() return only 0 (success) or a
negative error value, as pci_enable_device() and pci_disable_device() do.

Previously they returned:

    0:	device was already active (or disabled)
    1:	we just activated (or disabled) device
    <0:	-EBUSY or error from pnp_start_dev() (or pnp_stop_dev())

Now we return only 0 (device is active or disabled) or <0 (error).

All in-tree callers either ignore the return values or check only for
errors (negative values).

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Cc: Adam Belay <ambx1@neo.rr.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>