ixgbe: fix the FWSM.PT check in ixgbe_mng_present()

Bits other than FWSM.PT can be set in IXGBE_SWFW_MODE_MASK making the
previous check invalid.

Change the check for MNG present to be only based on FWSM.PT bit.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>