USB: serial: io_edgeport: 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 called while or after the port
had been closed. The return value was later changed to zero by commit
d76f2f44 ("io_edgeport: Fix various bogus returns to the tty
layer") but the bogus sanity checks were left in place as were the
outdated function-header comments.

These callbacks will never be called for an uninitialised port 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the unnecessary sanity checks and the outdated comments.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>