staging:iio:max1363: Don't free uninitialized variable

It is possible that on one of the error paths we are going to try to free
'rxbuf', even though it has not been allocated yet, which cause the following
warning:
	drivers/staging/iio/adc/max1363_ring.c: In function 'max1363_trigger_handler':
		drivers/staging/iio/adc/max1363_ring.c:87:7: warning: 'rxbuf' may be used
		uninitialized in this function

Reported-by: Fengguang Wu <wfg@linux.intel.com>
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>