modem: hl7800: sock_read len arg is not used

[rerickson1]

The static function sock_read does not need a length arg, remove it.

fixes #58566

[str4t0m]

As len is only assigned once, but never used, maybe even make it more explicit and cast the return value to void?

(void)net_buf_findctrl(*buf, &frag);

I am not sure if Coverity also warns about "assigned but never used".

[str4t0m]

I am going to close issue #67972, as it is an duplicate of the issue #58566 you are referencing. If this differs from the usual handling of duplicates please let me know.