spi/atmel: fix simple_return.cocci warnings

drivers/spi/spi-atmel.c:1518:1-4: WARNING: end returns can be simpified and declaration on line 1514 can be dropped

 Simplify a trivial if-return sequence.  Possibly combine with a
 preceding function call.
Generated by: scripts/coccinelle/misc/simple_return.cocci

Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>