The ACPI /proc write() code takes an unsigned length argument like any write()
function, but then assigned it to a *signed* integer called "len".
Only after this is a sanity check for len done to make it not larger than 4.

Due to the type change a len < 0 is in principle also possible; this patch
adds a check for this.
Signed-off-by: NArjan van de Ven <arjan@linux.intel.com>
Signed-off-by: NLen Brown <len.brown@intel.com>