brcmfmac: Split brcmf_sdiod_regrw_helper() up.

This large function is concealing a LOT of obscure logic about
how the hardware functions. Time to split it up.

This first patch splits the function into two pieces - read and write,
doing away with the rw flag in the process.

Signed-off-by: Ian Molton <ian@mnementh.co.uk>
Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>