xhci: fix SCT_FOR_CTX(p) macro

SCT_FOR_CTX(p) is defined as (((p) << 1) & 0x7) in which case if we want
to set the stream context type to SCT_SSA_256 i.e 0x7 (although secondary
stream arrays are not yet supported) using this macro definition we will
get actually 0x6 which is not what we want.

This patch fixes the above issue by defining the SCT_FOR_CTX(p) macro as
(((p) & 0x7) << 1)

Signed-off-by: Xenia Ragiadakou <burzalodowa@gmail.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>