tty: hso: simplify hso_serial_write()

tty: hso: simplify hso_serial_write()

There is no need for two more variables in hso_serial_write(). Switch to
min_t() and eliminate those.

Furthermore, the 'if-goto' is superfluous as memcpy() of zero count is a
nop. So is addition of zero. So remove the 'if-goto' completely.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Link: https://lore.kernel.org/r/20230810091510.13006-36-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>