n_tty: simplify and sanitize zero_buffer()

n_tty: simplify and sanitize zero_buffer()

* Make 'tty' parameter const as we only look at tty flags here.
* Make 'size' parameter of size_t type as everyone passes that and
  memset() (the consumer) expects size_t too. So be consistent.
* Remove redundant local variables, place the content directly to the
  'if'.
* Use 0 instead of 0x00 in memset(). The former is more obvious.

No functional changes expected.

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230712064216.12150-3-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>