cfi_flash: Use u8 pointers instead of void pointers

According to the C standard, pointer arithmetic for pointers of type
void is undefined behavior (the assumption that they're 8-bit wide is a
GCC-specific assumption). In the interest of keeping the code
standards-compliant, and also better communicate intent, switch all
void* variables where pointer arithmetic is used to u8* variables.

Signed-off-by: Mario Six <mario.six@gdsys.cc>
Signed-off-by: Stefan Roese <sr@denx.de>