net: ti: icssg-prueth: Fix signedness bug in prueth_init_tx_chns()

The "tx_chn->irq" variable is unsigned so the error checking does not
work correctly.

Fixes: 128d5874 ("net: ti: icssg-prueth: Add ICSSG ethernet driver")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Roger Quadros <rogerq@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>