brcmfmac: Replace function index with function pointer

In preparation for removing the function array, remove all code that
refers to function by index and replace with pointers to the function
itself.

Signed-off-by: Ian Molton <ian@mnementh.co.uk>
Reviewed-by: Arend van Spriel <arend.vanspriel@broadcom.com>
[arend: replace BUG() with WARN() macro]
Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>