esp.c: add FIFO wraparound support to esp_fifo_pop_buf()

esp.c: add FIFO wraparound support to esp_fifo_pop_buf()

The fifo8_pop_buf() function returns a pointer to the FIFO buffer up to the
specified length. Since the FIFO buffer is modelled as an array then once
the FIFO wraps around, only the continuous portion of the buffer can be
returned.

In future the use of continuous and unaligned accesses will advance the
internal FIFO head pointer, so modify esp_fifo_pop_buf() to ensure that
any wraparound content is also returned up to the requested length.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Tested-by: Helge Deller <deller@gmx.de>
Tested-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20240112125420.514425-4-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>