chardev/char-mux: shift unsigned long to avoid 32-bit overflow

chardev/char-mux: shift unsigned long to avoid 32-bit overflow

Allthough the size of MAX_MUX is equal to 4 and likely will never
change, this patch changes type of constant to unsigned long to
be on the safe side.

Also add a static compile check that MAX_MUX never bigger than
`sizeof(d->mux_bitset) * BITS_PER_BYTE`.

Signed-off-by: Roman Penyaev <r.peniaev@gmail.com>
Reviewed-by: "Marc-André Lureau" <marcandre.lureau@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Clément Mathieu--Drif <clement.mathieu--drif@eviden.com>
Cc: qemu-devel@nongnu.org
Message-ID: <20241129103239.464061-2-r.peniaev@gmail.com>