drivers/net/wireless/ath/wil6210: Makefile, only -Werror when no -W* in EXTRA_CFLAGS

  When make with EXTRA_CFLAGS=-W, it will report error.
  so give a check in Makefile.

Signed-off-by: Chen Gang <gang.chen@asianux.com>
Acked-by: Vladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>