ACPI: Fix a warning of discarding qualifiers from pointer target type

drivers/acpi/ec.c: In function `acpi_ec_ecdt_probe':
drivers/acpi/ec.c:873: warning: passing arg 1 of `acpi_get_devices' discards qualifiers from pointer target type

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Len Brown <len.brown@intel.com>