libsdhcdrivers: Rename mmc's transfer block size

`bs` was not descriptive enough, and `block_size` helps to avoid
confusion.

Further more variable is marked as const and assigned during the
declaration.

libsdhcdrivers/src/mmc.c

diff --git a/libsdhcdrivers/src/mmc.c b/libsdhcdrivers/src/mmc.c
index df7e849..bba9237 100644
--- a/libsdhcdrivers/src/mmc.c
+++ b/libsdhcdrivers/src/mmc.c
@@ -435,11 +435,12 @@
     uint32_t command)
 {
     struct mmc_cmd *cmd;
-    int bs;
-    bs = mmc_block_size(mmc_card);
+    const int block_size = mmc_block_size(mmc_card);
 
     /* Determine command argument */
-    const uint32_t arg = (mmc_card->high_capacity) ? start : (start * bs);
+    const uint32_t arg = (mmc_card->high_capacity)
+                         ? start
+                         : (start * block_size);
 
     /* Allocate command structure
      *
@@ -461,7 +462,7 @@
     struct mmc_completion_token *mmc_token = NULL;
 
     /* Add a data segment */
-    ret = mmc_cmd_add_data(cmd, vbuf, pbuf, start, bs, nblocks);
+    ret = mmc_cmd_add_data(cmd, vbuf, pbuf, start, block_size, nblocks);
     if (ret < 0) {
         goto exit_transfer_data;
     }
@@ -496,7 +497,7 @@
         }
     }
 
-    const size_t bytes_transferred = cb ? 0 : (bs * nblocks);
+    const size_t bytes_transferred = cb ? 0 : (block_size * nblocks);
     return is_success ? bytes_transferred : ret;
 }