fs:fat: fix set file name function

Curently memcpy copies string without null terminating char because
function strlen returns only number of characters excluding
null terminating character. Replace memcpy with strcpy.

Signed-off-by: Piotr Wilczek <p.wilczek@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
CC: Tom Rini <trini@ti.com>