rlc: Move prepare() function out of gprs_rlc_data struct

Newer gcc 10.1.0 is erroring due to memset being applied on a complex
type, so let's start by removing this only function outside of the
struct.

Change-Id: I20426557d9b3049ab275fadb92e10ea8a860a119
diff --git a/src/tbf_dl.cpp b/src/tbf_dl.cpp
index dab1e29..cd4ddb9 100644
--- a/src/tbf_dl.cpp
+++ b/src/tbf_dl.cpp
@@ -581,7 +581,7 @@
 
 	/* now we still have untransmitted LLC data, so we fill mac block */
 	rlc_data = m_rlc.block(bsn);
-	data = rlc_data->prepare(block_data_len);
+	data = prepare(rlc_data, block_data_len);
 	rlc_data->cs_last = cs;
 	rlc_data->cs_current_trans = cs;