app: fix volatile read for GCC >= 4.6

app: fix volatile read for GCC >= 4.6

When a memory address must be read, for instance a [mapped] PCI register,
the read value is assigned to a local variable that is not used after,
as for instance:
    x = *((uint8_t *) mem_addr);

Such instructions do not compile with gcc 4.6.
The fix consists in adding the "volatile" attribute to the accessed data type
and to not assign the read value:
    *((volatile uint8_t *) mem_addr);

Signed-off-by: Ivan Boule <ivan.boule@6wind.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>