qmi_wwan: Fix qmap header retrieval in qmimux_rx_fixup

[ Upstream commit d667044f49513d55fcfefe4fa8f8d96091782901 ]

This patch fixes qmap header retrieval when modem is configured for
dl data aggregation.

Signed-off-by: Daniele Palmas <dnlplm@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>