Bluetooth: Use common SOCK_STREAM receive code in RFCOMM

To reduce code duplication, have rfcomm_sock_recvmsg() call
bt_sock_stream_recvmsg().  The common bt_sock_stream_recvmsg()
code is nearly identical, with the RFCOMM-specific functionality
for deferred setup and connection unthrottling left in
rfcomm_sock_recvmsg().

Signed-off-by: Mat Martineau <mathewm@codeaurora.org>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>