lib: arm_ffa: Cast FF-A registers to the correct types

The FF-A fid in r0 and the error code for FFA_ERROR
should always be interpreted as an unsigned and signed
32-bit register, respectively. These values always get
passed in w0 and w2.

Bug: 284057071
Change-Id: I71460c722e7f1369b8a9e6ddbc9d803b3f3e0268