ACPI: Fix bound checks for copy_from_user in the acpi /proc code

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: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>