net: systemport: Enable all RX descriptors for SYSTEMPORT Lite

The original commit that added support for the SYSTEMPORT Lite variant
halved the number of RX descriptors due to a confusion between the
number of descriptors and the number of descriptor words. There are 512
descriptor *words* which means 256 descriptors total.

Fixes: 44a4524c

 ("net: systemport: Add support for SYSTEMPORT Lite")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20221007034201.4126054-1-f.fainelli@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>