IB: Remove __constant_{endian} uses

The base versions handle constant folding just fine, use them
directly.  The replacements are OK in the include/ files as they are
not exported to userspace so we don't need the __ prefixed versions.

This patch does not affect code generation at all.

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>