Revert "Rename all ACX_IE_* to ACX_REG_*: we talk about registers here"

Revert "Rename all ACX_IE_* to ACX_REG_*: we talk about registers here"

This reverts commit 56a8d0bea0c46530541afbdca8fad13387a9f58f.

Conflicts:

	acx_mmio.h
	usb.c

There is no need to refactor IE to REG. IE refers to Information
Elements, which was the original name for these logical structures.
Unless there is a solid reason against it, they should retain their
original name, regardless of their implementation (e.g. memory-mapped
registers). There are also some functions and references throughout the
code which make reference to these Information Elements, so we should be
careful about renaming.