caif: fix integer underflow in cffrml_receive()

In the Linux kernel, the following vulnerability has been resolved:

caif: fix integer underflow in cffrml_receive()

The cffrml_receive() function extracts a length field from the packet

header and, when FCS is disabled, subtracts 2 from this length without

validating that len >= 2.

If an attacker sends a malicious packet with a length field of 0 or 1

to an interface with FCS disabled, the subtraction causes an integer

underflow.

This can lead to memory exhaustion and kernel instability, potential

information disclosure if padding contains uninitialized kernel memory.

Fix this by validating that len >= 2 before performing the subtraction.