ixgbe: cleanup ixgbe_get_drvinfo to be extra careful with buffer boundaries

Make the code in ixgbe_get_drvinfo more protected against buffer overflow
boundaries by using snprintf and sizeof with snprintf.

Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>