davinci_mdio.c uses mdio bitbang APIs, so it should select
MDIO_BITBANG to prevent build errors.

arm-linux-gnueabi-ld: drivers/net/ethernet/ti/davinci_mdio.o: in function `davinci_mdio_remove':
drivers/net/ethernet/ti/davinci_mdio.c:649: undefined reference to `free_mdio_bitbang'
arm-linux-gnueabi-ld: drivers/net/ethernet/ti/davinci_mdio.o: in function `davinci_mdio_probe':
drivers/net/ethernet/ti/davinci_mdio.c:545: undefined reference to `alloc_mdio_bitbang'
arm-linux-gnueabi-ld: drivers/net/ethernet/ti/davinci_mdio.o: in function `davinci_mdiobb_read':
drivers/net/ethernet/ti/davinci_mdio.c:236: undefined reference to `mdiobb_read'
arm-linux-gnueabi-ld: drivers/net/ethernet/ti/davinci_mdio.o: in function `davinci_mdiobb_write':
drivers/net/ethernet/ti/davinci_mdio.c:253: undefined reference to `mdiobb_write'

Fixes: d04807b8 ("net: ethernet: ti: davinci_mdio: Add workaround for errata i2329")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Grygorii Strashko <grygorii.strashko@ti.com>
Cc: Ravi Gunasekaran <r-gunasekaran@ti.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Naresh Kamboju <naresh.kamboju@linaro.org>
Cc: Sudip Mukherjee (Codethink) <sudipm.mukherjee@gmail.com>
Link: https://lore.kernel.org/r/20220824024216.4939-1-rdunlap@infradead.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>