arm64: zynqmp: spl: use given boot_device instead of fetching it again

The boot_device argument to spl_boot_mode was massively added without
actually modifying the existing functions.

This commit actually makes use of the handed value, which is the same.

Signed-off-by: Jean-Francois Dagenais <jeff.dagenais@gmail.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>