staging: gdm72xx: WIMAX_GDM72XX_USB_PM should depend on USB_SUSPEND

Use Kconfig logic instead of a rude #error in the driver source,
which breaks m68k-allmodconfig.
Also remove the "default n", as that's the default.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Sage Ahn <syahn@gctsemi.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>