Spell out Hafnium in full, as specified by style guide.

Change-Id: Ib78170c1b1c53d39a2543a8bbd5d12ceaf24342e
diff --git a/src/api.c b/src/api.c
index d2cdd11..e7fb9cd 100644
--- a/src/api.c
+++ b/src/api.c
@@ -1419,7 +1419,7 @@
  *  In case of error one of the following values is returned:
  *   1) SPCI_INVALID_PARAMETERS - The endpoint provided parameters were
  *     erroneous;
- *   2) SPCI_NO_MEMORY - Hf did not have sufficient memory to complete
+ *   2) SPCI_NO_MEMORY - Hafnium did not have sufficient memory to complete
  *     the request.
  *  Success is indicated by SPCI_SUCCESS.
  */