Revert "tty: fix NULL pointer issue when tty_port ops is not set"

This reverts commit f4e68d58.

TTY drivers using the tty-port abstraction all provide a pointer to a
set of port operations, which specifically cannot be NULL (or we'd find
out at first attempt to open a port).

Revert the recent commit which added unnecessary NULL-checks and whose
commit message indicated that it was fixing a real problem, which it did
not.

Note that even the two tty drivers for virtual devices currently
providing an empty set of operations probably should be implementing at
least some of the callbacks.

Cc: Fabien Dessenne <fabien.dessenne@st.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>