tty: xtensa/iss: Use min() to fix Coccinelle warning

Inline strlen() and use min() to fix the following Coccinelle/coccicheck
warning reported by minmax.cocci:

	WARNING opportunity for min()

Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Message-Id: <20240404075811.6936-3-thorsten.blum@toblux.com>
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>