drivers: modem: gsm: remove use of preprocessor directives for rssi

The rssi_work_handle always exists in the gsm struct, so use

```
if (IS_ENABLED(CONFIG_GSM_MUX))
```
instead of

```
#if defined(CONFIG_GSM_MUX)
```

for better code readability.

Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>