ath11k: avoid null pointer dereference when pointer band is null

In the unlikely event that cap->supported_bands has neither
WMI_HOST_WLAN_2G_CAP set or WMI_HOST_WLAN_5G_CAP set then pointer
band is null and a null dereference occurs when assigning
band->n_iftype_data.  Move the assignment to the if blocks to
avoid this.  Cleans up static analysis warnings.

Addresses-Coverity: ("Explicit null dereference")
Fixes: 9f056ed8 ("ath11k: add HE support")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>