staging: rtl8723bs: Fix possible usage of NULL pointer in core/rtw_debug.c

Smatch reports the following:

  CHECK   drivers/staging/rtl8723bs/core/rtw_debug.c
drivers/staging/rtl8723bs/core/rtw_debug.c:454 proc_get_survey_info() error: we previously assumed 'phead' could be null (see line 453)
drivers/staging/rtl8723bs/core/rtw_debug.c:455 proc_get_survey_info() warn: variable dereferenced before check 'phead' (see line 454)

In the code, there are two successive calls to get_head(). The second
is removed.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>