staging: exfat: fix uninitialized variable ret

Currently there are error return paths in ffsReadFile that
exit via lable err_out that return and uninitialized error
return in variable ret. Fix this by initializing ret to zero.

Addresses-Coverity: ("Uninitialized scalar variable")
Fixes: c48c9f7f ("staging: exfat: add exfat filesystem code to staging")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Cc: Valdis Kletnieks <valdis.kletnieks@vt.edu>,
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Link: https://lore.kernel.org/r/20190830184644.15590-1-colin.king@canonical.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>