staging: exfat: stopusing CONFIG_FAT_DEFAULT_IOCHARSET

staging: exfat: stopusing CONFIG_FAT_DEFAULT_IOCHARSET

When CONFIG_VFAT_FS is disabled, the reference to CONFIG_FAT_DEFAULT_IOCHARSET
causes a link failure:

drivers/staging/exfat/exfat_super.c:46:41: error: use of undeclared identifier 'CONFIG_FAT_DEFAULT_IOCHARSET'
static char exfat_default_iocharset[] = CONFIG_FAT_DEFAULT_IOCHARSET;

I could not figure out why the correct code is commented
out, but using that fixes the problem.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20190906150917.1025250-1-arnd@arndb.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>