usb.h: fix ioctl kernel-doc info

Fix struct field name, prevent kernel-doc warnings.

Warning(include/linux/usb.h:865): No description found for parameter 'unlocked_ioctl'
Warning(include/linux/usb.h:865): Excess struct/union/enum/typedef member 'ioctl' description in 'usb_driver'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>