staging: sm750fb: Cleanup the type of mmio750

This patch assigns the more appropriate void* type to the mmio750 variable
eliminating an unnecessary volatile qualifier in the process. Additionally it
updates parameter types as necessary where those parameters interact with
mmio750, removes unnecessary casts and updates the type of the
lynx_share->pvReg field which is passed to the ddk750_set_mmio method.

As a consequence, this patch fixes the following sparse warning:-

drivers/staging/sm750fb/ddk750_help.c:12:17: warning: incorrect type in assignment (different address spaces)

Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>