cmd: sound: fix help text

There's never been a -q or -s argument handled in the command, so let's
remove it. This was highlighted during review[1] but somehow still got
through.

While at it, slightly "reword" in the help text how the len + freq
arguments are defined. Indeed, len and freq work in pair, it is possible
to define none of either, n of both, or n - 1 of freq if there are n
len, in which case the freq that goes with the last len would be the n -
1 (and not the default of 400Hz if neither len nor freq is passed). I
assume this isn't what's expected but leaving it for another patch if
need be to fix what happens in that very odd scenario.

[1] https://lore.kernel.org/u-boot/CAPnjgZ0QWNqVFZfEWHxRcFOA3E3gRAZCYs77nGUXKL0pLp+JLQ@mail.gmail.com/

Fixes: ea58b9a404d4 ("cmd: allow sound command to play multiple sounds")
Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Simon Glass <sjg@chromium.org>