spi/dln2: simplify return flow for dln2_spi_transfer_setup and dln2_spi_enable

This fixes the following kbuild test robot warnings:

>> drivers/spi/spi-dln2.c:124:1-4: WARNING: end returns can be simplified if negative or 0 value
>> drivers/spi/spi-dln2.c:656:1-4: WARNING: end returns can be simplified if negative or 0 value

Additionally, fix a comment after switching from CONFIG_PM_RUNTIME to
CONFIG_PM.

Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Laurentiu Palcu <laurentiu.palcu@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>