brcmfmac: Fix asymmetric IO functions.

Unlikely to be a problem, but brcmf_sdiod_regrl() is
not symmetric with brcmf_sdiod_regrb() in initializing
the data value on stack. Fix that.

Signed-off-by: Ian Molton <ian@mnementh.co.uk>
[arend: reword the commit message a bit]
Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>