ALSA: bebob: fix to detect configured source of sampling clock for Focusrite Saffire...

ALSA: bebob: fix to detect configured source of sampling clock for Focusrite Saffire Pro i/o series

For Focusrite Saffire Pro i/o, the lowest 8 bits of register represents
configured source of sampling clock. The next lowest 8 bits represents
whether the configured source is actually detected or not just after
the register is changed for the source.

Current implementation evaluates whole the register to detect configured
source. This results in failure due to the next lowest 8 bits when the
source is connected in advance.

This commit fixes the bug.

Fixes: 15889e4ae011 ("ALSA: bebob: Add support for Focusrite Saffire/SaffirePro series")
Cc: <stable@vger.kernel.org> # v3.16+
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20191102150920.20367-1-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Iwai <tiwai@suse.de>