iwlwifi: convert to use simple_open()

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

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>