drivers: staging: fbtft: fbtft-bus.c: Fix different address space warning on I/O mem

To fix sparse warning of incorrect type in assignment
(different address space), added annotation __iomem to
vmem8 and modified direct reads with ioread8().

Signed-off-by: Tolga Ceylan <tolga.ceylan@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>