hwmon: (fam15h_power) Fix unintentional integer overflow

Expression with two unsigned integer variables is calculated as unsigned integer
before it is converted to u64. This may result in an integer overflow.
Fix by typecasting the left operand to u64 before performing the left shift.

This patch addresses Coverity #402320: Unintentional integer overflow.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Jean Delvare <khali@linux-fr.org>
Acked-by: Andreas Herrmann <andreas.herrmann3@amd.com>