drivers: i2c_nrfx_twim: restore previous behavior without concat buffer

The Nordic TWIM peripheral generates a start condition for each bus
transaction.  Devices such as the SSD1306 display and some NXP sensors
can only tolerate the presence of a start condition and device address
after a stop condition.  Those devices will not operate correctly when
these signals are observed while the bus is already active.  This
motivated the addition of a RAM buffer into which message fragments
could be collected so TWIM can transmit them without injecting
unnecessary start conditions.

However many I2C devices interpret these signals as a repeated start
and ignore them and so function properly without a buffer
concatenating the message fragments.

There is no default for the concat-buf-size property, and the previous
strict requirement for one when performing scatter/gather I/O
transactions broke working drivers for devices that tolerate the
repeated starts.  Allow those drivers to work by respecting the
property description and attempting to concatenate messages only if a
buffer in which to place them has been provided.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>