usb: gadget: Kconfig: use bool instead of boolean

Keyword 'boolean' for type definition attributes is considered
deprecated and, therefore, should not be used anymore.

See http://lkml.kernel.org/r/cover.1418003065.git.cj@linux.com
See http://lkml.kernel.org/r/1419108071-11607-1-git-send-email-cj@linux.com



Signed-off-by: Christoph Jaeger <cj@linux.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>