Correct callback function signature `fn data_ptr(&self) -> *mut c_void`

[Taowyoo]

From @MihirLuthra :

I see we have some code in rust-mbedtls that would generally result in undefined behaviour. I mean the callback functions where we have signature like:
fn data_ptr(&self) -> *mut c_void
which usually results in:
self as *const _ as *mut _
Is there any ticket already to address this in mbedtls? Maybe changing the signature or making call sites use UnsafeCell