net: txgbe: Fix I2C Kconfig dependencies

The dependency handling of the Synopsys DesignWare I2C
adapter drivers is going to be changed so that the glue
drivers for the platform and PCI buses depend on
I2C_DESIGNWARE_CORE. Right now this driver prevents that
update because it selects I2C_DESIGNWARE_PLATFORM.

To make the dependency on I2C_DESIGNWARE_PLATFORM consistent
with the other drivers in kernel that depend on it, and
allow the dependency handling of the Synopsys DesignWare I2C
drivers to be updated, change the "select" into "depends on".

Cc: Jiawen Wu <jiawenwu@trustnetic.com>
Cc: Mengyuan Lou <mengyuanlou@net-swift.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: netdev@vger.kernel.org
Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Acked-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>