staging:iio:ad2s90: Make probe handle spi_setup failure

Previously, ad2s90_probe ignored the return code from spi_setup, not
handling its possible failure. This patch makes ad2s90_probe check if
the code is an error code and, if so, do the following:

- Call dev_err with an appropriate error message.
- Return the spi_setup's error code.

Note: The 'return ret' statement could be out of the 'if' block, but
this whole block will be moved up in the function in the patch:
'staging:iio:ad2s90: Move device registration to the end of probe'.

Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>