brcmfmac: Rename buscore to core for consistency

Avoid confusion with unrelated _buscore labels.

Signed-off-by: Ian Molton <ian@mnementh.co.uk>
Acked-by: Arend van Spriel <arend.vanspriel@broadcom.com>
[arend: only do the rename]
Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>