drivers: i2c: i2c_nrfx_twim: remove redundant buffer size from config

There are two different i2c node properites `zephyr,flash-buf-max-size`
and `zephyr,concat-buf-size`. In the end max value of that two is used
to define size of the message buffer.
It's redundant to store both values in device config structure.
Changed config structure to contain only bigger value.

Signed-off-by: Adam Wojasinski <adam.wojasinski@nordicsemi.no>