hwmon: ltc2991: fix register bits defines

In the LTC2991, V5 and V6 channels use the low nibble of the
"V5, V6, V7, and V8 Control Register" for configuration, but currently,
the high nibble is defined.

This patch changes the defines to use the low nibble.

Fixes: 2b9ea426 ("hwmon: Add driver for ltc2991")
Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>
Message-ID: <20240830111349.30531-1-paweldembicki@gmail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>