net: lan743x: select FIXED_PHY

The blamed commit introduces usage of fixed_phy_register() but
not a corresponding dependency on FIXED_PHY.

This can result in a build failure.

 s390-linux-ld: drivers/net/ethernet/microchip/lan743x_main.o: in function `lan743x_phy_open':
 drivers/net/ethernet/microchip/lan743x_main.c:1514: undefined reference to `fixed_phy_register'

Fixes: 624864fb

 ("net: lan743x: add fixed phy support for LAN7431 device")
Cc: stable@vger.kernel.org
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Closes: https://lore.kernel.org/netdev/725bf1c5-b252-7d19-7582-a6809716c7d6@infradead.org/
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Signed-off-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>