ARM: pxa: ssp: Check return values from phandle lookups

Commit a6e56c28

 (ARM: pxa: ssp: add DT bindings) causes warnings
when built:

arch/arm/plat-pxa/ssp.c: In function 'pxa_ssp_probe':
arch/arm/plat-pxa/ssp.c:145:17: warning: 'dma_spec.args[0]' may be used
	uninitialized in this function [-Wmaybe-uninitialized]

Resolve by checking return values and aborting when lookups fail.

Cc: Daniel Mack <zonque@gmail.com>
Cc: Mark Brown <broonie@linaro.org>
Cc: Haojian Zhuang <haojian.zhuang@gmail.com>
Signed-off-by: Olof Johansson <olof@lixom.net>