USB: serial: mos7720: drop buffer-callback sanity checks

The driver write_room and chars_in_buffer callbacks used to incorrectly
return a negative errno in case they were ever called with a NULL port
driver-data pointer. The return value was later changed to zero by
commit 23198fda ("tty: fix chars_in_buffers") but the bogus sanity
checks were left in place as were the outdated function-header comments.

The port driver data isn't cleared until after the port has been
deregistered and all open ttys have been hung up so drop the unnecessary
sanity checks and the outdated comments.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>