mwifiex: Use helper function for counting interface types

Use a small helper function to increment and decrement the counter of
the interface types we currently manage. This makes the code that
actually changes and sets up the interface type a bit less messy and
also helps avoiding mistakes in case someone increments/decrements a
counter wrongly.

Signed-off-by: Jonas Dreßler <verdre@v0yd.nl>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210914195909.36035-5-verdre@v0yd.nl