cxgb4: allocate more space for MSI-X interrupt names

Currently MSI-X names for netdevs with long names are truncated in
/proc/interrupts due to insufficient space.  Use IFNAMSIZ to size the
needed space.

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