wl1251: Update wl->nvs_len after wl->nvs is valid

If kmemdup fails, then wl->nvs_len will contain invalid non-zero size.

Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>