esp: remove the buf and buflen parameters from get_cmd()

Now that all SCSI commands are accumulated in cmdbuf, remove the buf and buflen
parameters from get_cmd() since these always reference cmdbuf and ESP_CMDBUF_SZ
respectively.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20210304221103.6369-21-mark.cave-ayland@ilande.co.uk>