net: bgmac: clarify CONFIG_BCMA dependency

The bgmac driver depends on BCMA_HOST_SOC, which is only used
when CONFIG_BCMA is enabled. However, it is a bool option and can
be set when CONFIG_BCMA=m, and then bgmac can be built-in, leading
to an obvious link error:

drivers/built-in.o: In function `bgmac_init':
:(.init.text+0x7f2c): undefined reference to `__bcma_driver_register'
drivers/built-in.o: In function `bgmac_exit':
:(.exit.text+0x110a): undefined reference to `bcma_driver_unregister'

To avoid this case, we need to depend on both BCMA and BCMA_SOC,
as this patch does. I'm also trying to make the dependency more
readable by splitting it into three lines, and adding a COMPILE_TEST
alternative so we can test-build it in all configurations that
support BCMA.

The added dependency on FIXED_PHY addresses a related issue where
we cannot call fixed_phy_register() when CONFIG_FIXED_PHY=m and
CONFIG_BGMAC=y.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>