The current prototype of new_mmio_info() uses void* for parameters read
and write, which are functions with precise calling conventions
(argument types and return type). Write down these conventions in
new_mmio_info() definition.

This has been reported by the following warnings when clang is used to
build the kernel:

    drivers/gpu/drm/i915/gvt/handlers.c:124:21: error: pointer type
    mismatch ('void *' and 'int (*)(struct intel_vgpu *, unsigned int,
    void *, unsigned int)') [-Werror,-Wpointer-type-mismatch]
            info->read = read ? read : intel_vgpu_default_mmio_read;
                              ^ ~~~~   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    drivers/gpu/drm/i915/gvt/handlers.c:125:23: error: pointer type
    mismatch ('void *' and 'int (*)(struct intel_vgpu *, unsigned int,
    void *, unsigned int)') [-Werror,-Wpointer-type-mismatch]
            info->write = write ? write : intel_vgpu_default_mmio_write;
                                ^ ~~~~~   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This allows the compiler to detect that sbi_ctl_mmio_write() returns a
"bool" value instead of an expected "int" one. Fix this.
Signed-off-by: Nicolas Iooss <nicolas.iooss_linux@m4x.org>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>