iio: iio_event_getfd -- fix ev_int build failure

Fix build failure in staging iio driver:

.../drivers/staging/iio/industrialio-core.c: In function 'iio_event_getfd':
.../drivers/staging/iio/industrialio-core.c:262:32: error:
			'ev_int' undeclared (first use in this function)

Also convert the rest of the function to use the new variable.

Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>