net: dsa: LAN9303: select REGMAP when LAN9303 enable

When NET_DSA_SMSC_LAN9303=y and NET_DSA_SMSC_LAN9303_MDIO=y,
below errors can be seen:
drivers/net/dsa/lan9303_mdio.c:87:23: error: REGMAP_ENDIAN_LITTLE
undeclared here (not in a function)
  .reg_format_endian = REGMAP_ENDIAN_LITTLE,
drivers/net/dsa/lan9303_mdio.c:93:3: error: const struct regmap_config
has no member named reg_read
  .reg_read = lan9303_mdio_read,

It should select REGMAP in config NET_DSA_SMSC_LAN9303.

Fixes: dc700583 ("net: dsa: LAN9303: add MDIO managed mode support")
Signed-off-by: Mao Wenan <maowenan@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>