USB: serial: mos7840: drop buffer-callback return-value comments

The driver write_room and chars_in_buffer callbacks used to incorrectly
return a negative errno in case they were called with a NULL port
driver-data pointer or if some other always-true sanity checks failed.

The bogus sanity checks were later removed by commit ce039bd4b21f ("USB:
serial: mos7840: drop paranoid port checks") and 7b2faede671a ("USB:
serial: mos7840: drop port driver data accessors") but the
function-header comments were never updated to match.

Drop the outdated return-value comments.

Reviewed-by: Greg Kroah-Hartman <[e-mail]>
Signed-off-by: Johan Hovold <[e-mail]>

drivers/usb/serial/mos7840.c

···
  *	this function is called by the tty driver when it wants to know how many
  *	bytes of data we currently have outstanding in the port (data that has
  *	been written, but hasn't made it out the port yet)
- *	If successful, we return the number of bytes left to be written in the
- *	system,
- *	Otherwise we return zero.
  *****************************************************************************/
 
 static unsigned int mos7840_chars_in_buffer(struct tty_struct *tty)
···
  * mos7840_write_room
  *	this function is called by the tty driver when it wants to know how many
  *	bytes of data we can accept for a specific port.
- *	If successful, we return the amount of room that we have for this port
- *	Otherwise we return a negative error number.
  *****************************************************************************/
 
 static unsigned int mos7840_write_room(struct tty_struct *tty)