trusty: Avoid signed overflow

SMC_FC_FFA_MEM_FRAG_RX returns the 16 bit sender-id in the top 16 bits
of 32 bit register. Cast to uint32_t before shifting it up to avoid
having the uint16_t promoted to a signed int for the shift.

Currently the value we return will never have the top bit of the
sender-id in this context, but this could change in the future.

Bug: 185307117
Change-Id: I401e3bcf9dbe2ffa99f03e527df0f9f3f8ff0d3e
1 file changed