vpx_ports/mem_ops.h: cast the lhs of bitwise shifts of 24.

C does not allow for shifting into the sign bit of a signed
integer, and the two instances here become signed ints via
promotion. Explcitly cast them to unsigned MEM_VALUE_T to
avoid the problem.

BUG=https://bugs.chromium.org/p/chromium/issues/detail?id=614648

Change-Id: I51165361a8c6cbb5c378cf7e4e0f4b80b3ad9a6e
1 file changed