staging: mt7621-pci: factor out 'mt7621_pcie_enable_port' function

Driver probe function is a mess and shall be refactored a lot. At first
make use of assert and deassert control factoring out a new function
called 'mt7621_pcie_enable_port'.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>