trace: events: scm: Use correct size when copying argument buffer

When copying the SCM call arugment buffer for trace events,
the amount of bytes specified to copy is MAX_SCM_ARGS, which
is not correct, as this is only the maximum number of SCM
call arguments, but each argument is 8 bytes, and thus the
amount of bytes that should be copied is actually 80 bytes.

Since the size of the SCM call argument buffer is a compile
time constant, use sizeof() on the argument buffer to compute
the amount of bytes that should be copied.

Change-Id: Ib878006e60d5bf283ba8252498c651b3e3c99b28
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>