esp: convert cmdbuf from array to Fifo8

Rename ESP_CMDBUF_SZ to ESP_CMDFIFO_SZ and cmdbuf_cdb_offset to cmdfifo_cdb_offset
to indicate that the command buffer type has changed from an array to a Fifo8.

This also enables us to remove the ESPState field cmdlen since the command length
is now simply the number of elements used in cmdfifo.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20210304221103.6369-40-mark.cave-ayland@ilande.co.uk>