USB: Don't loop endlessly waiting for a physical buffer.

This is used quite a lot in critical transfer paths, so we don't
want to lock things up if no buffers are available for whatever reason.
Wait 2 seconds, and if we didn't get anything by then, return B_NO_MEMORY.

Possibly fixes or helps with certain USB-related lockups.

src/add-ons/kernel/bus_managers/usb/PhysicalMemoryAllocator.cpp

@@ -222,7 +222,10 @@ PhysicalMemoryAllocator::Allocate(size_t size, void **logicalAddress,
 		TRACE_ERROR(("PMA: found no free slot to store %ld bytes, waiting\n",
 			size));
 
-		entry.Wait();
+		if (entry.Wait(B_RELATIVE_TIMEOUT, 2 * 1000 * 1000) == B_TIMED_OUT) {
+			TRACE_ERROR(("PMA: timed out waiting for a free slot, giving up\n"));
+			break;
+		}
 
 		if (!_Lock())
 			return B_ERROR;