wlcore/wl12xx/wl18xx: configure num_links per-hw

Upcoming fw versions will have different max links support
(according to the hw). Get ready for it by configuring
wl->num_links per-hw, instead of using the const WL12XX_MAX_LINKS.

However, continue using WLCORE_MAX_LINKS in order to simplify
structs declarations (we use it in multiple bitmaps, and converting
them to dynamic arrays is just cumbersome).

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>