brcmfmac: fix wrong strnchr usage

strnchr takes arguments in the order of its name: string, max bytes to
read, character to search for. Here we're passing '\n' aka 10 as the
buffer size, and searching for sizeof(buf) aka BRCMF_DCMD_SMLEN aka
256 (aka '\0', since it's implicitly converted to char) within those 10
bytes.

Just interchanging the last two arguments would still leave a bug,
because if we've been successful once, there are not sizeof(buf)
characters left after the new value of p.

Since clmver is immediately afterwards passed as a %s argument, I assume
that it is actually a properly nul-terminated string. For that case, we
have strreplace().

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>