fs: fat: Fix mkcksum() function parameters

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>