hwmon: (pmbus/acbel-fsg032) Add firmware version debugfs attribute

Like the IBM CFFPS driver, export the PSU's firmware version to a
debugfs attribute as reported in the manufacturer register.

Signed-off-by: Eddie James <eajames@linux.ibm.com>
Link: https://lore.kernel.org/r/20230628153453.122213-1-eajames@linux.ibm.com


[groeck: Dropped unused variable; changed buffer from char to u8]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>