edge: Provide and use CS -> CPS conversion

The MS' RLC receiver needs a valid CPS field to decode the block.

Add the function gprs_rlc_mcs_cps to generate the CPS value based on
coding scheme, puncturing, and padding.

Sponsored-by: On-Waves ehf
diff --git a/src/rlc.h b/src/rlc.h
index a11b4ce..76af4e1 100644
--- a/src/rlc.h
+++ b/src/rlc.h
@@ -105,6 +105,7 @@
 	GprsCodingScheme cs);
 void gprs_rlc_data_block_info_init(struct gprs_rlc_data_block_info *rdbi,
 	GprsCodingScheme cs);
+unsigned int gprs_rlc_mcs_cps(GprsCodingScheme cs, int punct, int with_padding);
 
 /*
  * I hold the currently transferred blocks and will provide