powerpc/qe: Fix Kconfig enablement of QE_USB support

Commit 193ab2a6 changed the USB gadget
Kconfig symbol from USB_GADGET_FSL_QE to USB_FSL_QE, but did not update
the associated symbol name in qe_lib to match.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>