hwmon: (pcf8591) Fix checkpatch issues

Fixed:
ERROR: do not use assignment in if condition
ERROR: spaces required around that ':' (ctx:VxV)
ERROR: spaces required around that '?' (ctx:VxV)
WARNING: line over 80 characters
WARNING: simple_strtoul is obsolete, use kstrtoul instead

Modify multi-line comments to follow Documentation/CodingStyle.

Cc: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>