staging:iio: Fix compile error without CONFIG_DEBUG_FS

commit e553f182 ("staging: iio: core: Introduce debugfs support, add support
for direct register access") added a '#if defined(CONFIG_DEBUG_FS)' around
iio_read_channel_ext_info and iio_write_channel_ext_info causing the following
compile error if CONFIG_DEBUG_FS is not defined.

	drivers/staging/iio/industrialio-core.c:621:11: error: 'iio_read_channel_ext_info' undeclared (first use in this function)
	drivers/staging/iio/industrialio-core.c:623:11: error: 'iio_write_channel_ext_info' undeclared (first use in this function)

This patch fixes the issue by moving the functions out of the '#if
defined(CONFIG_DEBUG_FS)' section again.

Reported-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>