commit 5c32d123 upstream.

N_TTY's direct and flow-controlled flavors of the .receive_buf()
method are nearly identical; fold together.
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[ kamal: 3.13-stable prereq for
  fb5ef9e7 n_tty: Fix read buffer overwrite when no newline ]
Cc: Joseph Salisbury <joseph.salisbury@canonical.com>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>