ppp: clean up endianness conversions

ppp: clean up endianness conversions

sparse complains about some harmless endianness issues:

> drivers/net/ppp/pptp.c:281:21: warning: incorrect type in assignment (different base types)
> drivers/net/ppp/pptp.c:281:21:    expected unsigned int [usertype] ack
> drivers/net/ppp/pptp.c:281:21:    got restricted __be32
> drivers/net/ppp/pptp.c:283:23: warning: cast to restricted __be32

Here 'ack' is assigned a value in network-order, and then also the
byte-swapped value in host-order. Clean this up by doing the byte-swap
as part of the assignment.

> drivers/net/ppp/pptp.c:358:26: warning: cast from restricted __be16
> drivers/net/ppp/pptp.c:358:26: warning: incorrect type in argument 1 (different base types)
> drivers/net/ppp/pptp.c:358:26:    expected unsigned short [usertype] call_id
> drivers/net/ppp/pptp.c:358:26:    got restricted __be16 [usertype]

Here we use the wrong flavour of byte-swap. Use ntohs(), which of course
gives the same result.

Cc: Dmitry Kozlov <xeb@mail.ru>
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Link: https://lore.kernel.org/r/20210107143956.25549-1-jwi@linux.ibm.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>