ste_dma40: Delete an unnecessary variable initialisation in d40_probe()

The variable "res" will eventually be set to a resource pointer from
a call of the d40_hw_detect_init(() function.
Thus let us omit the explicit initialisation at the beginning.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>