net: thunderx: Improvement for MBX interface debug messages

Adding debug messages in case of NACK for a mailbox message, also
did small cleanups.

Signed-off-by: Radoslaw Biernacki <rad@semihalf.com>
Signed-off-by: Sunil Goutham <sgoutham@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>