net: ipa: use common value for channel type and protocol

The numeric values that represent the event ring channel type are
identical to the values that represent the matching protocol used
for a channel.  Use a new gsi_channel_type enumerated type to
represent the values programmed for both cases, using "CHANNEL_TYPE"
in member names in place of "EVT_CHTYPE" and "CHANNEL_PROTOCOL".

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>