ALSA: firewire-lib: use macro for maximum value of second in 1394 OHCI isoc descriptor

ALSA: firewire-lib: use macro for maximum value of second in 1394 OHCI isoc descriptor

In descriptor of isochronous context in 1394 OHCI, the field of second
has 3 bit, thus the maximum value is 8. The value is used for correct
cycle calculation.

This commit replaces hard-coded value with macro to obsolete magic
number.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20200508043635.349339-3-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Iwai <tiwai@suse.de>