wifi: brcmfmac: do not cast hidden SSID attribute value to boolean

In 'brcmf_cfg80211_start_ap()', not assume that
NL80211_HIDDEN_SSID_NOT_IN_USE is zero but prefer
an explicit check instead. Compile tested only.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Signed-off-by: Alexey Berezhok <a@bayrepo.ru>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://msgid.link/20240208085121.2430-1-a@bayrepo.ru