ACPI: replace strlen("string") with sizeof("string") -1

...both give the number of chars in the string
without the '\0', as strncmp() wants,
but sizeof() is compile-time.

Reported-by: Alan Stern <stern@rowland.harvard.edu>
Cc: Pavel Vasilyev <pavel@pavlinux.ru>
Signed-off-by: Len Brown <len.brown@intel.com>