hw/drivers/i2s: Fix i2s_write
hw/drivers/i2s: Fix i2s_write

Synchronous function for easy write to i2s base speaker
was not setting up sample_count field.
If this filed was not setup in other way (as it could
potentially be) i2s drivers would just drop empty
buffers and no sound would emitted.

mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core