sdio: add sdio_f0_readb() and sdio_f0_writeb()

Add sdio_f0_readb() and sdio_f0_writeb() functions to reading and
writing function 0 registers.  Writes outside the vendor specific CCCR
registers (0xF0 - 0xFF) are not permitted.

Signed-off-by: David Vrabel <david.vrabel@csr.com>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>