wifi: brcmsmac: replace deprecated strncpy with memcpy

Let's move away from using strncpy and instead use the more obvious
interface for this context.

For wlc->pub->srom_ccode, we're just copying two bytes from ccode into
wlc->pub->srom_ccode with no expectation that srom_ccode be
NUL-terminated:
wlc->pub->srom_ccode is only used in regulatory_hint():
1193 |       if (wl->pub->srom_ccode[0] &&
1194 |           regulatory_hint(wl->wiphy, wl->pub->srom_ccode))
1195 |               wiphy_err(wl->wiphy, "%s: regulatory hint failed\n", __func__);

We can see that only index 0 and index 1 are accessed.
3307 |       int regulatory_hint(struct wiphy *wiphy, const char *alpha2)
3308 |       {
...  |          ...
3322 |          request->alpha2[0] = alpha2[0];
3323 |          request->alpha2[1] = alpha2[1];
...  |          ...
3332 |       }

Since this is just a simple byte copy with correct lengths, let's use
memcpy(). There should be no functional change.

In a similar boat, both wlc->country_default and
wlc->autocountry_default are just simple byte copies so let's use
memcpy. However, FWICT they aren't used anywhere. (they should be
used or removed -- not in scope of my patch, though).

Signed-off-by: Justin Stitt <justinstitt@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20231017-strncpy-drivers-net-wireless-broadcom-brcm80211-brcmfmac-cfg80211-c-v3-2-af780d74ae38@google.com