net: express the first argument to NetSetTimeout() in milliseconds

Enforce millisecond semantics of the first argument to NetSetTimeout() --
the change is transparent for well-behaving boards (CFG_HZ == 1000 and
get_timer() countiing in milliseconds).

Rationale for this patch is to enable millisecond granularity for
network-related timeouts, which is needed for the upcoming automatic
software update feature.

Summary of changes:
- do not scale the first argument to NetSetTimeout() by CFG_HZ
- change timeout values used in the networking code to milliseconds

Signed-off-by: Rafal Czubak <rcz@semihalf.com>
Signed-off-by: Bartlomiej Sieka <tur@semihalf.com>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>