Use `is_connected` to determine whether a ctx/rtx was connected to uarte

[hdoordt]

Uarte::free method uses bit_is_set to determine whether a pin was connected to cts and rts. However, a set bit represents a disconnected pin. Use is_connected() instead to do the inversion.

[jonas-schievink]

Thanks!

bors r+

[bors[bot]]

Build succeeded:

• ci