bus/fpga: use strlcpy instead of strncpy

bus/fpga: use strlcpy instead of strncpy

Gcc 8 reports a build error:

drivers/bus/ifpga/ifpga_bus.c:161:2: error: ‘strncpy’ specified
bound 256 equals destination size [-Werror=stringop-truncation]
  strncpy(afu_pr_conf.bs_path, path, sizeof(afu_pr_conf.bs_path));
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Fixes: 05fa3d4a6539 ("bus/ifpga: add Intel FPGA bus library")

Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>