lm4549: Fix buffer overflow

Report from smatch:
lm4549.c:234 lm4549_write_samples(14) error:
 buffer overflow 's->buffer' 1024 <= 1024

There must be enough space to add two entries starting with index
s->buffer_level, therefore the old check was wrong.

[Peter Maydell <peter.maydell@linaro.org> clarifies the nature of the
analyser warning:

I don't object to making the change to placate the analyser,
but I don't think this is actually a buffer overrun. We always
add and remove samples from the buffer two at a time, so it's
not possible to get here with s->buffer_level == BUFFER_SIZE-1
(which is the only case where the old and new conditions
give different answers).]

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>