net: bcmdhd: remove gcc 7 build warnings

net: bcmdhd: remove gcc 7 build warnings

Remove following gcc 7 build warnings:
   drivers/net/wireless/bcmdhd/dhd_common.c: In function ‘dhd_pktfilter_offload_set’:
   drivers/net/wireless/bcmdhd/dhd_common.c:3015:16: error: comparison between pointer and zero character constant [-Werror=pointer-compare]
       if (argv[i] == '\0') {
                   ^~
   drivers/net/wireless/bcmdhd/dhd_common.c:3015:8: note: did you mean to dereference the pointer?
       if (argv[i] == '\0') {
           ^
   drivers/net/wireless/bcmdhd/dhd_rtt.c: In function ‘dhd_rtt_init’:
   drivers/net/wireless/bcmdhd/dhd_rtt.c:983:56: error: ‘subcmd_info.version’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
     *out_version = (ret == BCME_OK) ? subcmd_info.version : 0;
                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
   drivers/net/wireless/bcmdhd/dhd_rtt.c:977:20: note: ‘subcmd_info.version’ was declared here
     ftm_subcmd_info_t subcmd_info;
                       ^~~~~~~~~~~

Change-Id: I5990eaba2272ecb9633bb0058668eb426fc99d2e
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>