tty: make counts in tty_port_client_operations hooks size_t

tty: make counts in tty_port_client_operations hooks size_t

The counts in tty_port_client_operations hooks' are currently
represented by all 'int', 'unsigned int', and 'size_t'. Unify them all
to unsigned 'size_t' for clarity. Note that size_t is used already in
tty_buffer.c. So, eventually, it is spread for counts everywhere and
this is the beginning.

So the two changes namely:
* ::receive_buf() is called from tty_ldisc_receive_buf(). And that
  expects values ">= 0" from ::receive_buf(), so switch its rettype to
  size_t is fine. tty_ldisc_receive_buf() types will be changed
  separately.
* ::lookahead_buf()'s count comes from lookahead_bufs() and is already
  'unsigned int'.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Cc: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230810091510.13006-11-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>