usb: dwc3: fix wrong bit mask in dwc3_event_type

Per dwc3 2.50a spec, the is_devspec bit is used to distinguish the
Device Endpoint-Specific Event or Device-Specific Event (DEVT). If the
bit is 1, the event is represented Device-Specific Event, then use
[7:1] bits as Device Specific Event to marked the type. It has 7 bits,
and we can see the reserved8_31 variable name which means from 8 to 31
bits marked reserved, actually there are 24 bits not 25 bits between
that. And 1 + 7 + 24 = 32, the event size is 4 byes.

So in dwc3_event_type, the bit mask should be:
is_devspec	[0]		1  bit
type		[7:1]		7  bits
reserved8_31	[31:8]		24 bits

This patch should be backported to kernels as old as 3.2, that contain
the commit 72246da4 "usb: Introduce
DesignWare USB3 DRD Driver".

Cc: <stable@vger.kernel.org>
Signed-off-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>