soc: ti/ti_sci_protocol.h: drop a duplicated word + clarify

Drop the repeated word "an" in a comment.
Insert "and" between "source" and "destination" as is done a few
lines earlier.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>