r600: Make enum alu_op_flags unsigned

r600: Make enum alu_op_flags unsigned

In builds with clang, there are several errors related to the enum
alu_op_flags like this:

src/gallium/drivers/r600/sb/sb_expr.cpp:887:8:
error: case value evaluates to -1610612736, which cannot be narrowed to
type 'unsigned int' [-Wc++11-narrowing]

These are due to the MSB being set in the enum. Fix these errors by
making the enum values unsigned as needed. The flags field that stores
this enum also needs to be unsigned.

Cc: "11.1 11.2" <mesa-stable@lists.freedesktop.org>
Cc: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>