staging: ks7010: use u8 instead of unsigned char in write_to_device function

Parameter buffer in write_to_device function is declared as
a pointer to unsigned char and can be declared as an u8 type
which is preferred. Internally it calls to ks7010_sdio_write
which is using also u8 as parameter type. Update calls to this
function as well.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>