fs: fat: convert change month correctly

The month is stored in 5 - 8. We need to shift it by 5 bits.

Cf. Microsoft FAT Specification, 2005-08-30

Fixes: 13c11c66 ("fs: fat: add file attributes to struct fs_dirent")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Alexander Dahl <ada@thorsis.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>