xen: xen-pciback: remove DRIVER_ATTR() usage

It's better to be explicit and use the DRIVER_ATTR_RW() and
DRIVER_ATTR_RO() macros when defining a driver's sysfs file.

Bonus is this fixes up a checkpatch.pl warning.

This is part of a series to drop DRIVER_ATTR() from the tree entirely.

Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>