fix(rss): do not consider MHU_ERR_ALREADY_INIT as error

rss_comms_init() should return with success (0) in case of an internal
MHU_ERR_ALREADY_INIT error code which is harmless (occurs when
rss_comms_init() is called multiple times in a row).

Change-Id: Ibb1fef48a60866e80d3a389128cb8eef1332ea01
Signed-off-by: David Vincze <david.vincze@arm.com>