MIPS: generic: Don't explicitly disable CONFIG_USB_SUPPORT

Leave CONFIG_USB_SUPPORT at its default, allowing board config fragments
to make use of USB drivers without needing to override it & trigger
warnings from merge_config.sh.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/16948/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>