caif-hsi: changed test on aggregation_timeout

Aggregation_timeout is an unsigned long,
a test for less than zero can never become true,
compare with zero instead.

Signed-off-by: Kim Lilliestierna <kim.xx.lilliestierna@stericsson.com>
Signed-off-by: Sjur Brændeland <sjur.brandeland@stericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>