i2c/eeprom: Always define I2C_RXTX_LEN

I2C_RXTX_LEN from include/i2c.h is not defined if CONFIG_DM_I2C is
enabled. This leads to a compilation error on boards that enable both
CONFIG_CMD_EEPROM and CONFIG_DM_I2C.

To avoid this, we define I2C_RXTX_LEN in cmd/eeprom.c if it is not
already defined.

Signed-off-by: Mario Six <mario.six@gdsys.cc>