net: ipa: clarify endpoint register macro constraints

A handful of registers are valid only for RX endpoints, and some
others are valid only for TX endpoints.  For these endpoints, add
a comment above their defined offset macro that indicates the
endpoints to which they apply.

Extend the endpoint parameter naming convention as well, to make
these constraints more explicit.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>