ec_host_cmd: shi: npcx: change log level for some error messages

It was observed in a project that when an error transaction occurs,
printing the error message can delay resetting the SHI state machine and
initializing the output buffer, which may cause the subsequent
transaction to fail, especially during stress tests involving heavy host
command communication.
This commit changes some error log level from ERR to DBG to prevent the
condition above from happening.
Also change the default debug level for the SHI driver to LOG_LEVEL_ERR.

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