usb: ulpi: kconfig: Remove meaningless choice default

usb: ulpi: kconfig: Remove meaningless choice default

'default' on a choice refers to the symbol selected by default, not to
the choice mode, so 'default n' is meaningless.

No functional changes. Optional choices implicitly default to n mode
(and there is no way to make them default to another mode).

Discovered in Kconfiglib (https://github.com/ulfalizer/Kconfiglib),
which prints the following warning:

	warning: the default selection n (undefined) of <choice> (defined at drivers/usb/ulpi/Kconfig:3) is not contained in the choice

I've added a corresponding warning to the C tools too, which is
currently in linux-next: https://patchwork.kernel.org/patch/9983667/

Signed-off-by: Ulf Magnusson <ulfalizer@gmail.com>