brcmfmac: Remove brcmf_sdiod_request_data()

This function is obfuscating how IO works on this chip. Remove it
and push its logic into brcmf_sdiod_reg_{read,write}().

Handling of -ENOMEDIUM is altered, but as that's pretty much broken anyway
we can ignore that.

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>