The mci-bcm2835 driver registers itself as an MCI driver on probing, and
is thereby used through the MCI interface, so these functions are not
meant to act as a public interface and only used internally in this
compilation unit.

This fixes the following build warnings:

    .../drivers/mci/mci-bcm2835.c:56:6: warning: no previous prototype for 'bcm2835_mci_write' [-Wmissing-prototypes]
     void bcm2835_mci_write(struct bcm2835_mci_host *host, u32 reg, u32 val)
          ^~~~~~~~~~~~~~~~~
    .../drivers/mci/mci-bcm2835.c:74:5: warning: no previous prototype for 'bcm2835_mci_read' [-Wmissing-prototypes]
     u32 bcm2835_mci_read(struct bcm2835_mci_host *host, u32 reg)
         ^~~~~~~~~~~~~~~~
    .../drivers/mci/mci-bcm2835.c:83:6: warning: no previous prototype for 'bcm2835_mci_write_data' [-Wmissing-prototypes]
     void bcm2835_mci_write_data(struct bcm2835_mci_host *host, u32 *p)
          ^~~~~~~~~~~~~~~~~~~~~~
    .../drivers/mci/mci-bcm2835.c:89:6: warning: no previous prototype for 'bcm2835_mci_read_data' [-Wmissing-prototypes]
     void bcm2835_mci_read_data(struct bcm2835_mci_host *host, u32 *p)
          ^~~~~~~~~~~~~~~~~~~~~
      CC      common/date.o
    .../drivers/mci/mci-bcm2835.c:419:5: warning: no previous prototype for 'bcm2835_mci_reset' [-Wmissing-prototypes]
     int bcm2835_mci_reset(struct mci_host *mci, struct device_d *mci_dev)

Signed-off-by: Roland Hieber <rhi@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>