chardev/char-mux: make boolean bit check instead of find_next_bit()

chardev/char-mux: make boolean bit check instead of find_next_bit()

This patch simplifies (and makes less confusing) bit checks by
replacing `find_next_bit()` calls with boolean AND operation.

Resolves: Coverity CID 1563776
Signed-off-by: Roman Penyaev <r.peniaev@gmail.com>
Reviewed-by: "Marc-André Lureau" <marcandre.lureau@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Clément Mathieu--Drif <clement.mathieu--drif@eviden.com>
Cc: qemu-devel@nongnu.org
Message-ID: <20241129103239.464061-3-r.peniaev@gmail.com>