pseries/papr-hvpipe: Refactor and simplify hvpipe_rtas_recv_msg()

pseries/papr-hvpipe: Refactor and simplify hvpipe_rtas_recv_msg()

Simplify hvpipe_rtas_recv_msg() by removing three levels of nesting...
if (!ret)
    if (buf)
	if (size < bytes_written)
... this refactoring of the function bails out to "out:" label first, in case
of any error. This simplifies the init flow.

Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/bbe7ddf8b8e25c9be8fc5e2c4aea9e5fca128bf4.1777606826.git.ritesh.list@gmail.com