usb: serial: mos7840: Fixup mos7840_chars_in_buffer()

 * Use the buffer content length as opposed to the total buffer size.  This can
   be a real problem when using the mos7840 as a usb serial-console as all
   kernel output is truncated during boot.

Signed-off-by: Mark Ferrell <mferrell@uplogix.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>