1<<31 (same as 2<<30) is undefined behaviour in C. When compiling with
GCC and UBSan, it gives this error:

../tools/intel_reg_decode.c: In function ‘ivb_debug_port’:
../tools/intel_reg_decode.c:398:3: error: case label does not reduce to an integer constant
   case PORT_DBG_DRRS_HW_STATE_HIGH:
   ^~~~

This happens because 1<<31 isn't representable as a signed int. Instead,
use an unsigned int.

Signed-off-by: Simon Ser <simon.ser@intel.com>
Reviewed-by: Petri Latvala <petri.latvala@intel.com>