rtw89: fix uninitialized variable of rtw89_append_probe_req_ie()

smatch reports that:
  fw.c:1997 rtw89_append_probe_req_ie() error: uninitialized symbol 'ret'.

This can be a issue only if no band is supported by the chip, but it is
impossible. So, it is still safe without this patch.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220311020007.20414-1-pkshih@realtek.com