With CONFIG_I2C=m and NET_DSA_SMSC_LAN9303=y, we run into a link error:

drivers/base/regmap/regmap-i2c.o: In function `regmap_smbus_byte_reg_read':
regmap-i2c.c:(.text.regmap_smbus_byte_reg_read+0x18): undefined reference to `i2c_smbus_read_byte_data'
drivers/base/regmap/regmap-i2c.o: In function `regmap_smbus_byte_reg_write':
regmap-i2c.c:(.text.regmap_smbus_byte_reg_write+0x18): undefined reference to `i2c_smbus_write_byte_data'
drivers/base/regmap/regmap-i2c.o: In function `regmap_smbus_word_reg_read':
regmap-i2c.c:(.text.regmap_smbus_word_reg_read+0x18): undefined reference to `i2c_smbus_read_word_data'
drivers/base/regmap/regmap-i2c.o: In function `regmap_smbus_word_read_swapped':
regmap-i2c.c:(.text.regmap_smbus_word_read_swapped+0x18): undefined reference to `i2c_smbus_read_word_data'
drivers/base/regmap/regmap-i2c.o: In function `regmap_smbus_word_write_swapped':

This adds a Kconfig dependency to avoid the broken configuration.

Fixes: be4e119f ("net: dsa: LAN9303: add I2C managed mode support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>