drivers: i2c: npcx: enhance the transfer efficiency

The i2c_transfer() API allows multiple msgs objects to be carried in a
transaction. After handling the current msgs object in the interrupt
context, the driver notifies the calling thread to buffer the next
msgs object and generate the Re-Start if required.
However, if the calling thread is preempted by higher priority threads,
the I2C transaction time may become non-deterministic (depending on the
execution time of higher-priority threads). This commits modifies the
driver to handle msgs objects entirely in the interrupt context to
improve the I2C transfer efficiency..

Signed-off-by: Jun Lin <CHLin56@nuvoton.com>