drivers/net/airoha_eth: enable hw padding of short tx packets

drivers/net/airoha_eth: enable hw padding of short tx packets

Transmission of short packets does not work good for XFI (GDM2) and
HSGMII (GDM3) interfaces. The issue can be solved with:

 - padding of short packets to 60 bytes
 - setting of PAD_EN bit in the corresponding REG_GDM_FWD_CFG(n)
   register.

The issue should present for the lan switch (GDM1) as well, but it does
does not appear due to unknown reason.

This patch set PAD_EN bit for the used GDM.

Signed-off-by: Mikhail Kshevetskiy <mikhail.kshevetskiy@iopsys.eu>