fs: fat: Fix mkcksum() function parameters

The mkcksum() function now takes one parameter, the pointer to
11-byte wide character array, which it then operates on.

Currently, the function is wrongly passed (dir_entry)->name, which
is only 8-byte wide character array. Though by further inspecting
the dir_entry structure, it can be noticed that the name[8] entry
is immediatelly followed by ext[3] entry. Thus, name[8] and ext[3]
in the dir_entry structure actually work as this 11-byte wide array
since they're placed right next to each other by current compiler
behavior.

Depending on this is obviously wrong, thus fix this by correctly
passing both (dir_entry)->name and (dir_entry)->ext to the mkcksum()
function and adjust the function appropriately.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Tom Rini <trini@ti.com>
diff --git a/fs/fat/fat_write.c b/fs/fat/fat_write.c
index 5829adf..4a1bda0 100644
--- a/fs/fat/fat_write.c
+++ b/fs/fat/fat_write.c
@@ -335,7 +335,7 @@
 
 	/* Get short file name and checksum value */
 	strncpy(s_name, (*dentptr)->name, 16);
-	checksum = mkcksum(s_name);
+	checksum = mkcksum((*dentptr)->name, (*dentptr)->ext);
 
 	do {
 		memset(slotptr, 0x00, sizeof(dir_slot));