xilinx: zynqmp: change the type of multiboot variable

xilinx: zynqmp: change the type of multiboot variable

In function ‘set_dfu_alt_info’ a comparison of a u8 value against
0 is done. Since it is always false, change the signature of this
function to use an `int` instead, which match the type used in caller:
`multi_boot()`.

Fix the following warning triggered with W=1:

board/xilinx/zynqmp/zynqmp.c:651:23:
warning: comparison is always false due to limited range of data type
[-Wtype-limits]
651 |         if (multiboot < 0)

Signed-off-by: Venkatesh Yadav Abbarapu <venkatesh.abbarapu@amd.com>
Link: https://lore.kernel.org/r/20221004055254.26246-1-venkatesh.abbarapu@amd.com
Signed-off-by: Michal Simek <michal.simek@amd.com>