net: ipa: use "tre_ring" for all TRE ring local variables

All local variables that represent event rings are named "ring".

All but two functions that represent a channel's TRE ring with a
local variable use the name "tre_ring".  For consistency, use that
name in the two functions that don't fit the pattern.

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