net: systemport: Correct IPG length settings

Due to a documentation mistake, the IPG length was set to 0x12 while it
should have been 12 (decimal). This would affect short packet (64B
typically) performance since the IPG was bigger than necessary.

Fixes: 44a4524c ("net: systemport: Add support for SYSTEMPORT Lite")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>