usb_gigaset: don't kmalloc(0)

Zero-sized allocations are pointless anyway, and the SLUB allocator
complains about them, so stop doing that.

Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: Hansjoerg Lipp <hjlipp@web.de>
Cc: Greg KH <greg@kroah.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>