net: gemini: remove unnecessary assert

The driver does not advertize NETIF_F_FRAGLIST, the stack can't
pass skbs with frags lists to the xmit function.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>