When B53_SERDES is a loadable module, a built-in srab driver still
cannot reach it, so the previous fix is incomplete:

b53_srab.c:(.text+0x3f4): undefined reference to `b53_serdes_init'
drivers/net/dsa/b53/b53_srab.o:(.rodata+0xe64): undefined reference to `b53_serdes_link_state'
drivers/net/dsa/b53/b53_srab.o:(.rodata+0xe74): undefined reference to `b53_serdes_link_set'
drivers/net/dsa/b53/b53_srab.o:(.rodata+0xe88): undefined reference to `b53_serdes_an_restart'
drivers/net/dsa/b53/b53_srab.o:(.rodata+0xea0): undefined reference to `b53_serdes_phylink_validate'
drivers/net/dsa/b53/b53_srab.o:(.rodata+0xea4): undefined reference to `b53_serdes_config'

Add a Kconfig dependency that forces srab to also be a module
in this case, but allow it to be built-in when serdes is
disabled or built-in.

Fixes: 7a8c7f5c ("net: dsa: b53: Fix build with B53_SRAB enabled and not B53_SERDES")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>