wireless: bcmdhd4361: fix fix comparing a pointer to a null character

drivers/net/wireless/bcmdhd4361/dhd_common.c:3839:19: error: comparing a pointer to a null character constant; did you mean to compare to N
ULL? [-Werror,-Wpointer-compare]
                        if (argv[i] == '\0') {
                                       ^~~~
                                       (void *)0

Change-Id: I4b6613761ff3965cb8741046d4d62d2ea04a852e