ixgbe: use checker safe conversions

The ixgbe hardware needs some very specific programming for
certain registers, which led to some misguided usage of ntohs
instead of using be16_to_cpu(), as well as a home grown swap
followed by an ntohs. Sparse didn't like this at all, and this
fixes the C=2 build, with code that uses native kernel interface.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Dave Switzer <david.switzer@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>