PCI: Rename sysfs 'enabled' file back to 'enable'

Back in commit 5136b2da ("PCI: convert bus code to use dev_groups"),
I misstyped the 'enable' sysfs filename as 'enabled', which broke the
userspace API.  This patch fixes that issue by renaming the file back.

Fixes: 5136b2da ("PCI: convert bus code to use dev_groups")
Reported-by: Jeff Epler <jepler@unpythonic.net>
Tested-by: Jeff Epler <jepler@unpythonic.net>	# on v3.14-rt
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
CC: stable@vger.kernel.org	# 3.13