Fix all fo the bugs I introduced "improving" the code of others.

mmc_ll_cmds.c: mmc_mode_select and mmc_get_configuration now work.
mmc_hl_cmds.c: bug introduced by turning a var into a pointer to that variable.

lib/driver/mmc/mmc_cmd_helper.h

@@ -33,7 +33,6 @@
     if ( ! p_cdio ) return DRIVER_OP_UNINIT;				\
     if ( ! p_cdio->op.run_mmc_cmd ) return DRIVER_OP_UNSUPPORTED;	\
 									\
-    memset (p_buf, 0, i_size);						\
     CDIO_MMC_SET_COMMAND(cdb.field, mmc_cmd)				
 
 /* Boilerplate initialization code to setup running MMC read command