net: mqtt: Debug logging of pointers

Cast pointer to `void *` for `%p` parameter.
Otherwise lots of warnings in the log like below:

 `<wrn> cbprintf_package: (unsigned) char * used for %p argument.
 It's recommended to cast it to void * because it may cause
 misbehavior in certain configurations. String:"%s: (%p): >>
 length:0x%08x cur:%p, end:%p" argument:3`

Signed-off-by: Jun Qing Zou <jun.qing.zou@nordicsemi.no>