Replace UNPACK_CB64 macro with inline

Having submitted a module based on coreboot to LKML for acceptance,
it was requested that fewer macros and more inlines be used (because
of their superior type-checking when performing pointer casts, etc).

This is the first of several changes to make the relevant parts of
coreboot comply to linux code standards.

Change-Id: Iffe7061fa62fa639e0cb6ccb9125eb3403d06b1a
Signed-off-by: Philip Prindeville <philipp@redfish-solutions.com>
Reviewed-on: http://review.coreboot.org/495
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>