esp.c: rework esp_cdb_length() into esp_cdb_ready()

esp.c: rework esp_cdb_length() into esp_cdb_ready()

The esp_cdb_length() function is only used as part of a calculation to determine
whether the cmdfifo contains an entire SCSI CDB. Rework esp_cdb_length() into a
new esp_cdb_ready() function which both enables us to handle the case where
scsi_cdb_length() returns -1, plus simplify the logic for its callers.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20240324191707.623175-12-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>