i2c: target: eeprom_target: Fix buffer write

When larger buffer index was introduced only function:
eeprom_target_write_received() was updated to handle
address-width = 16

This adds the same functionality when buffered API is used,
enabled by CONFIG_I2C_TARGET_BUFFER_MODE.

Signed-off-by: Jerzy Kasenberg <jerzy.kasenberg@codecoup.pl>