net: context: Refactor option getters

Set separate option getters for bool, uint8_t and uint16_t
values. Use those generic getters when fetching the desired
option value.

Noticed mixed usage (bool vs int) for txtime option. Changed
the code to use int type like in other options.

The uint16_t option getter gets the value from uint16_t variable
but returns int value to the caller, and also expects that user
supplies int value.

For uint8_t value, it is expected that uint8_t value is supplied
instead of int.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>