tests: Bluetooth: tester: Fix bt_micp_mic_ctlr_mute_get invalid usage

The call to bt_micp_mic_ctlr_mute_get function will return the value of
mute state in bt_micp_mic_ctlr_cb.mute callback. This fixes invalid
function calls by storing the recent mute state value in the
application.

Fixes: MICP/CL/SPE/BI-01-C
Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>