net/cadence_gem: Improve can_receive debug printfery

net/cadence_gem: Improve can_receive debug printfery

Currently this just floods indicating that can_receive has been called
by the net framework. Instead, save the result of the most recent
can_receive callback as state and only print a message if the result
changes (indicating some sort of actual state change in GEM). Make said
debug message more meaningful as well.

Signed-off-by: Peter Crosthwaite <[email protected]>
Message-id: 2eb74ca6a5756aea242d9f525961db95d6cfcf2c.1386136219[email protected]
Signed-off-by: Peter Maydell <[email protected]>