iio: adc: vf610: fix simple_return.cocci warnings

drivers/iio/adc/vf610_adc.c:766:1-4: WARNING: end returns can be simpified and declaration on line 755 can be dropped

 Simplify a trivial if-return sequence.  Possibly combine with a
 preceding function call.

Generated by: scripts/coccinelle/misc/simple_return.cocci

CC: Sanchayan Maity <maitysanchayan@gmail.com>
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>