net: ipa: rename channel->tlv_count

Each GSI channel has a TLV FIFO of a certain size, specified in the
configuration data for an AP channel.  That size dictates the
maximum number of TREs that are allowed in a single transaction.

The only way that value is used after initialization is as a limit
on the number of TREs in a transaction; calling it "tlv_count"
isn't helpful, and in fact gsi_channel_trans_tre_max() exists to
sort of abstract it.

Instead, rename the channel->tlv_count field trans_tre_max, and get
rid of the helper function.  Update a couple of comments as well.

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