libsdhcdrivers: Free allocated memory

If no callback is given, command must be destroyed prior to returning
from the function.

libsdhcdrivers/src/mmc.c

diff --git a/libsdhcdrivers/src/mmc.c b/libsdhcdrivers/src/mmc.c
index 2b1a4a6..85810a5 100644
--- a/libsdhcdrivers/src/mmc.c
+++ b/libsdhcdrivers/src/mmc.c
@@ -477,6 +477,7 @@
     } else {
         unsigned long ret;
         ret = host_send_command(mmc_card, cmd, NULL, NULL);
+        mmc_cmd_destroy(cmd);
         return (ret) ? ret : bs * nblocks;
     }
 }