ixgbe: Make ixgbe_identify_qsfp_module_generic static

Correct a namespace complaint by making the function static
and moving the prototype into the .c file.

Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>