Switch the printk() call to the prefered pr_warn() api.

Fixes: 7e5873d3 ("MIPS: pci: Add MT7620a PCIE driver")
Signed-off-by: John Crispin <john@phrozen.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Cc: <stable@vger.kernel.org> # 4.5+
Patchwork: https://patchwork.linux-mips.org/patch/15321/Signed-off-by: James Hogan <jhogan@kernel.org>