hwmon: (pmbus) Fix 'Macros with multiple statements' checkpatch error

Fix:
ERROR: Macros with multiple statements should be enclosed in a do - while loop

by unwinding the problematic macros.

As a side effect, this patch reduces code size on x86_64 by 160 bytes and bss
size by 64 bytes.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>