usb: xhci: Fix TRB transfer length macro used for Event TRB.

Use proper macro while extracting TRB transfer length from
Transfer event TRBs. Adding a macro EVENT_TRB_LEN (bits 0:23)
for the same, and use it instead of TRB_LEN (bits 0:16) in
case of event TRBs.

This patch should be backported to kernels as old as 2.6.31, that
contain the commit b10de142 "USB: xhci:
Bulk transfer support".  This patch will have issues applying to older
kernels.

Signed-off-by: Vivek gautam <gautam.vivek@samsung.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Cc: stable@vger.kernel.org