net: smc91c111: gate can_receive() on rx FIFO having a slot

Return false from can_receive() when the FIFO doesn't have a free RX
slot. This fixes a bug in the current code where the allocated buffer
is freed before the fifo pop, triggering a premature flush of queued RX
packets. It also will handle a corner case, where the guest manually
frees the allocated buffer before popping the rx FIFO (hence it is not
enough to just delay the flush_queued_packets()).

Reported-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Tested-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Message-id: 97bfdfc5cbce0bd5e0cbbbff35ce7a1bf6f8603d.1441873621.git.crosthwaite.peter@gmail.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>