ixgbe: correct register names to match datasheet

DPDK commit message

net/ixgbe/base: correct registers names to match datasheet
Some of mailbox-related registers have different names than it is
specified in datasheet. Correct these names to correspond to their
datasheet counterparts. Additionally, several calculations are changed
to no longer use magic numbers but dedicated macros instead.

Signed-off-by: Jakub Chylkowski <jakubx.chylkowski@intel.com>
Reviewed-by: Marek Zalfresso-jundzillo <marekx.zalfresso-jundzillo@intel.com>
Reviewed-by: Alice Michael <alice.michael@intel.com>
Reviewed-by: Piotr Skajewski <piotrx.skajewski@intel.com>
Reviewed-by: Slawomir Mrozowicz <slawomirx.mrozowicz@intel.com>
Tested-by: Piotr Skajewski <piotrx.skajewski@intel.com>
Tested-by: Alice Michael <alice.michael@intel.com>

Obtained from: DPDK (10fd55e)
MFC after: 1 week