usb: gadget: gsi: Set NUM_OF_TRB as 15 for IN and OUT GSI endpoints

It is recommended to use NUM_OF_TRB greater than USB controller's
TRB cache size. Currently USB controller's TRB cache size on GSI
available platform is 12. TRB ring base address must be aligned to
TRB ring length rounded to power of two. Hence set NUM_OF_TRB as
15 for both IN and OUT GSI endpoints.

CRs-Fixed: 1067833
Change-Id: Iab8b0bc6fd17183fc6056f06a90421cde24987c8
Signed-off-by: Mayank Rana <mrana@codeaurora.org>