kni: fix build clean on Ubuntu

kni: fix build clean on Ubuntu

The $(comma) variable is not defined in this Makefile, nor in
any included Makefile. Seen while doing a "make clean" on ubuntu:

  $ make clean
  == Clean lib
  == Clean lib/librte_compat
  == Clean lib/librte_eal
  == Clean lib/librte_eal/common
  == Clean lib/librte_eal/linuxapp
  == Clean lib/librte_eal/linuxapp/eal
  == Clean lib/librte_eal/linuxapp/igb_uio
  == Clean lib/librte_eal/linuxapp/kni
  tr: missing operand after ‘.-’
  Two strings must be given when translating.
  Try 'tr --help' for more information.

This commit replaces $(comma) by a ',' character, it's not a problem in
that case since we are inside antiquotes.

Fixes: a09b359daca3 ("kni: fix build on Ubuntu 14.04")

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by Ferruh Yigit <ferruh.yigit@intel.com>