Fix build error when CONFIG_SMP is turned off:

CC [M]  arch/mips/cavium-octeon/octeon-usb.o
arch/mips/cavium-octeon/octeon-usb.c: In function
‘dwc3_octeon_device_init’:
arch/mips/cavium-octeon/octeon-usb.c:540:4: error: implicit declaration
of function ‘devm_iounmap’ [-Werror=implicit-function-declaration]
     devm_iounmap(&pdev->dev, base);
Signed-off-by: Steven J. Hill <steven.hill@cavium.com>
Reviewed-by: James Hogan <james.hogan@imgtec.com>
Tested-by: Matt Redfearn <matt.redfearn@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/16907/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>