brcmfmac: fix size of the struct msgbuf_ring_status

This updates host struct to match the in-firmawre definition. It's a
cosmetic change as it only applies to the reserved struct space.

Fixes: c988b782 ("brcmfmac: print firmware reported ring status errors")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>