tipc: fix incorrect setting window for bcast link

In commit 16ad3f40
("tipc: introduce variable window congestion control"), we applied
the algorithm to select window size from minimum window to the
configured maximum window for unicast link, and, besides we chose
to keep the window size for broadcast link unchanged and equal (i.e
fix window 50)

However, when setting maximum window variable via command, the window
variable was re-initialized to unexpect value (i.e 32).

We fix this by updating the fix window for broadcast as we stated.

Fixes: 16ad3f40 ("tipc: introduce variable window congestion control")
Acked-by: Jon Maloy <jmaloy@redhat.com>
Signed-off-by: Hoang Huu Le <hoang.h.le@dektech.com.au>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>