staging/sm7xxfb: use module_pci_driver macro

as the manual of module_pci_driver says that
it can be used when the init and exit functions of
the module does nothing but the pci_register_driver
and pci_unregister_driver.

use it for the sm7xxfb driver, as the driver does nothing in
its _init and _exit functions but the register and unregister.

Signed-off-by: Devendra Naga <devendra.aaru@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>