net: wwan: iosm: fix incorrect skb length

skb passed to network layer contains incorrect length.

In mux aggregation protocol, the datagram block received
from device contains block signature, packet & datagram
header. The right skb len to be calculated by subracting
datagram pad len from datagram length.

Whereas in mux lite protocol, the skb contains single
datagram so skb len is calculated by subtracting the
packet offset from datagram header.

Fixes: 1f52d7b6

 ("net: wwan: iosm: Enable M.2 7360 WWAN card support")
Signed-off-by: M Chetan Kumar <m.chetan.kumar@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>