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 ce039bd4 ("USB:
serial: mos7840: drop paranoid port checks") and 7b2faede ("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Drop the outdated return-value comments.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 <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>