When sparse support was (intended to be) made optional it was
effectively unconditionally disabled because

	IS_ENABLED(USB_GADGET_FASTBOOT_SPARSE)

always evaluates to 0. To actually make use of the introduced kconfig
symbol the CONFIG_ prefix must not be skipped.

Fixes: f4b5d3eeb607 ("usb: gadget: fastboot: Make sparse support optional")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Tested-by: Gavin Schenk <g.schenk@eckelmann.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>