i2c: npcm: Add tx complete counter

tx_complete counter is used to indicate successful transaction count.
Similar counters for failed tx were previously added.

Signed-off-by: Tali Perry <tali.perry1@gmail.com>
Signed-off-by: Tyrone Ting <kfting@nuvoton.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>