lsdvb crashes if device is in use

lsdvb crashes if the device is in use (for example by tvheadend). Problem is the file mode. `O_RDWR` is not needed, `O_RDONLY` is enough.

Original output (actual):
```
$ lsdvb

                lsdvb: Simple utility to list PCI/PCIe DVB devices
                Version: 0.0.4
                Copyright (C) Manu Abraham

em28xx (632515409:0 0:1) on PCI Domain:31888 Bus:-818575984 Device:31888 Function:-818390219
        DEVICE:0 ADAPTER:0 ERROR: Open frontend0 failed
```

Expected output (with patch):
```
$ lsdvb

                lsdvb: Simple utility to list PCI/PCIe DVB devices
                Version: 0.0.4
                Copyright (C) Manu Abraham

em28xx (-633244093:0 0:1) on PCI Domain:32354 Bus:-2121079408 Device:32354 Function:-2120893643
        DEVICE:0 ADAPTER:0 FRONTEND:0 (Silicon Labs Si2168)
                 FE_QAM Fmin=48MHz Fmax=870MHz
        DEVICE:0 ADAPTER:1 FRONTEND:0 (Silicon Labs Si2168)
                 FE_QAM Fmin=48MHz Fmax=870MHz
```

patch:
```
--- linuxtv-dvb-apps-1.1.1+rev1500/util/lsdvb/lsdvb.c_orig 2025-11-08 23:01:10.000484848 +0000
+++ linuxtv-dvb-apps-1.1.1+rev1500/util/lsdvb/lsdvb.c 2025-11-08 23:04:33.941737821 +0000
@@ -290,7 +290,7 @@

    get_frontend_info(adapter, frontend);
    sprintf(fedev, "%s/adapter%d/%s", DVB_DIR, adapter, frontend);
- fd = open(fedev, O_RDWR | O_NONBLOCK);
+ fd = open(fedev, O_RDONLY | O_NONBLOCK);
    if (fd < 0) {
     fprintf(stderr, "ERROR: Open %s failed\n", frontend);
     return -1;

```

Ubuntu 24.04,

Package: dvb-apps
Architecture: amd64
Version: 1.1.1+rev1500-1.4build3