Staging: iio/light/tsl2563: fix compile warning

This fixes a compile warning:
drivers/staging/iio/light/tsl2563.c:696:2:
	warning: initialization from incompatible pointer type [enabled by default]
drivers/staging/iio/light/tsl2563.c:696:2:
	warning: (near initialization for ‘tsl2563_info.write_event_value’) [enabled by default]

The tsl2563_write_thresh() function returns zero on success and error
codes on failure, so nothing is lost by making the return type int
instead of ssize_t.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>