tty/serial: remove CONFIG_EXPERIMENTAL dependencies

As discussed at the kernel summit this year, CONFIG_EXPERIMENTAL means
nothing, so let's get rid of it.

Cc: Kees Cook <keescook@chromium.org>
Cc: Alan Cox <alan@linux.intel.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>