wifi: mwifiex: Fix uninitialized variable in mwifiex_cfg80211_authenticate()

Smatch complains that:

    drivers/net/wireless/marvell/mwifiex/cfg80211.c:4408 mwifiex_cfg80211_authenticate()
    error: uninitialized symbol 'varptr'.

It's a check for NULL, but "varptr" is either non-NULL or uninitialized.
Initialize it to NULL.

Fixes: 36995892 ("wifi: mwifiex: add host mlme for client mode")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Acked-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://patch.msgid.link/d7d043b2-95d5-4e1d-b340-5d7330053ac6@stanley.mountain