net: dsa: b53: Fix build with B53_SRAB enabled and B53_SERDES=m

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: 7a8c7f5c30f9 ("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>

drivers/net/dsa/b53/Kconfig

@@ -31,6 +31,7 @@ config B53_MMAP_DRIVER
 config B53_SRAB_DRIVER
 	tristate "B53 SRAB connected switch driver"
 	depends on B53 && HAS_IOMEM
+	depends on B53_SERDES || !B53_SERDES
 	default ARCH_BCM_IPROC
 	help
 	  Select to enable support for memory-mapped Switch Register Access