cxgb4: correct formatting of MSI-X interrupt names

The last byte of the buffer for MSI-X names could not be used due to a
bogus -1.  Also do not explicitly clear the last byte, snprintf will do
the right thing.

Signed-off-by: Dimitris Michailidis <dm@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>