net: lan966x: Fix duplicate check in frame extraction

The blamed commit generates the following smatch static checker warning:

 drivers/net/ethernet/microchip/lan966x/lan966x_main.c:515 lan966x_xtr_irq_handler()
         warn: duplicate check 'sz < 0' (previous on line 502)

This patch fixes this issue removing the duplicate check 'sz < 0'

Fixes: d28d6d2e ("net: lan966x: add port module support")
Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>