wifi: brcmfmac: avoid NULL-deref in survey dump for 2G only device

When dealing with a device for 2GHz band only the wiphy->bands for
5GHz will be NULL. This would result in a NULL-deref in the
brcmf_cfg80211_dump_survey() function. Rework the code with a
for-loop to make it easier to add another band.

Fixes: 6c04deae ("brcmfmac: Add dump_survey cfg80211 ops for HostApd AutoChannelSelection")
Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230103124117.271988-3-arend.vanspriel@broadcom.com