USB: g_printer: fix handling zero-length packet

g_printer doesn't have to check whether the data size is a multiple of
MaxPacketSize, because device controller driver already make that check.

Signed-off-by: SangSu <Park&lt;sangsu@gmail.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: David Brownell <david-b@pacbell.net>
Cc: Greg KH <greg@kroah.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>