wlcore/wl12xx/18xx: split fw_status struct into two

The number of RX packet descriptors may vary from chip to chip and
in different firmware versions.  Unfortunately, the array that
contains the actual descriptors is in the middle of the fw_status
structure.  To manage this, we split the struct into two so we can
calculate the offset of what comes after the array and access the last
elements more easily.

[Changed the STATUS_LEN macro to be placement agnostic - Arik]

Signed-off-by: Luciano Coelho <coelho@ti.com>
Signed-off-by: Arik Nemtsov <arik@wizery.com>