PCI: histb: Drop unnecessary root_bus_nr setting

Function dw_pcie_host_init() already initializes the root_bus_nr field
of 'struct pcie_port', so the -1 assignment prior to calling
dw_pcie_host_init() in platform specific driver is not really needed.
Drop it.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Jianguo Sun <sunjianguo1@huawei.com>