staging: clocking-wizard: Fix incorrect type of speed grade

The speed grade type is unsigned according to the dt-bindings.

Fix sparse issue (-Wtypesign):
CHECK   drivers/staging/clocking-wizard/clk-xlnx-clock-wizard.c
drivers/staging/clocking-wizard/clk-xlnx-clock-wizard.c:162:56: warning: incorrect type in argument 3 (different signedness)
drivers/staging/clocking-wizard/clk-xlnx-clock-wizard.c:162:56:    expected unsigned int [usertype] *out_value
drivers/staging/clocking-wizard/clk-xlnx-clock-wizard.c:162:56:    got int *<noident>

Signed-off-by: Xavier Roumegue <xroumegue@gmail.com>
Acked-by: Sören Brinkmann <soren.brinkmann@xilinx.com>
Acked-by: Moritz Fischer <moritz.fischer@ettus.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>