brcmfmac: Cleanup offsetof()

Create a macro to make the code a bit more readable, whilst we're stuck
with using struct element offsets as register offsets.

Signed-off-by: Ian Molton <ian@mnementh.co.uk>
Reviewed-by: Arend van Spriel <arend.vanspriel@broadcom.com>
[arend: rename macro to SD_REG]
Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>