ixgbe: Resolve cppcheck format string warning

cppcheck warns that the format string is incorrect in the function
ixgbe_get_strings().  Since the value cannot be negative, change the
variable to unsigned which matches the format specifier.

Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>