ASoC: max9759: fix underflow in speaker_gain_control_put()

In the Linux kernel, the following vulnerability has been resolved: ASoC: max9759: fix underflow in speaker_gain_control_put() Check for negative values of "priv->gain" to prevent an out of bounds access. The concern is that these might come from the user via: -> snd_ctl_elem_write_user() -> snd_ctl_elem_write() -> kctl->put()