net/mqtt: Use the right data type

Use the appropriate data type for some variables:

- const input arguments
- int32_t instead of uint32_t for timeout
- uint8_t instead of int for variables that take just a few
  possitive values.

Change-Id: Id7d762b400fa17e6751829ac401cd7c13aabb7b2
Signed-off-by: Flavio Santes <flavio.santes@intel.com>