eeprom: fix DM_I2C support without CONFIG_SYS_I2C_EEPROM_BUS

The current device model enabled eeprom code only works if
CONFIG_SYS_I2C_EEPROM_BUS is set.

This patch makes it work without that define so that the bus
number passed to 'eeprom_init' is used.

Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Reviewed-by: Heiko Schocher <hs@denx.de>