mwifiex: use 'total_ie_len' in mwifiex_update_bss_desc_with_ie()

This is clearer than copy/pasting the magic number '+ 2' around, and it
even saves the need for one existing comment.

Cc: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Brian Norris <briannorris@chromium.org>
Reviewed-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>