wil6210: fix parsing wigig.ini configuration file

Parsing the configuration file is using the function strsep to check
lines with parameter and followed by '=' and value.

The function char *strsep(char **stringp, const char *delim)
will return all stringp if the delim string was not found in stringp,
it will also set NULL for stringp.

Fix the code to check if stringp parameter to strsep was set to NULL.

Change-Id: I7ddbd175f8292e629f227ce9b6ba4d51e4b437da
Signed-off-by: Ahmad Masri <amasri@codeaurora.org>