wcn36xx: fix coccinelle warnings

drivers/net/wireless/ath/wcn36xx/debug.c:27:11-31: WARNING opportunity for simple_open, see also structure on line 106
/c/kernel-tests/src/i386/drivers/net/wireless/ath/wcn36xx/debug.c:27:11-31: WARNING opportunity for simple_open, see also structure on line 148

 This removes an open coded simple_open() function
 and replaces file operations references to the function
 with simple_open() instead.

Generated by: coccinelle/api/simple_open.cocci

CC: Eugene Krasnikov <k.eugene.e@gmail.com>
CC: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>