net: stmmac: loongson1: get ls1b resource only once

ls1b_dwmac_syscon_init() was getting the stmmac iomem resource to detect
which GMAC block is being used. Move this to a separate setup() function
that only runs at probe time, so it can sensibly behave with an
unrecognised resource adress. Use this to set a MAC index (id) which is
then used in place of testing the base address.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Keguang Zhang <keguang.zhang@gmail.com>
Tested-by: Keguang Zhang <keguang.zhang@gmail.com> # on LS1B & LS1C
Link: https://patch.msgid.link/E1uRqEE-004c7M-Go@rmk-PC.armlinux.org.uk


Signed-off-by: Jakub Kicinski <kuba@kernel.org>