Remove the assumption that pnp_register_driver() returns the number of devices
claimed.  Returning the count is unreliable because devices may be hot-plugged
in the future.

This changes the convention to "zero for success, or a negative error value,"
which matches pci_register_driver(), acpi_bus_register_driver(), and
platform_driver_register().
Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Cc: Adam Belay <ambx1@neo.rr.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>