usb: usbtest: fix the bit mask of usb 2.0 extension descriptor

USB 2.1 Link PM adds to use bits[1:15] according to USB 2.0 ECN Errata for
Link Power Management spec.

Bit	Encoding
0	Reserved
1	LPM
2	BESL & Altemate HIRD definitions supported
3	Recommended Baseline BESL valid
4	Recommended Deep BESL valid
11:8	Recommended Baseline BESL value
15:12	Recommended Deep BESL value
31:16	Reserved

So fix the bit mask from 0x1e to 0xfffe.

Reported-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Signed-off-by: Huang Rui <ray.huang@amd.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Acked-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>