usb: dwb_otg: Fix unreachable switch statement warning

This warning appears with GCC 7.3.0 from toolchains.bootlin.com:

../drivers/usb/host/dwc_otg/dwc_otg_fiq_fsm.c: In function ‘fiq_fsm_update_hs_isoc’:
../drivers/usb/host/dwc_otg/dwc_otg_fiq_fsm.c:595:61: warning: statement will never be executed [-Wswitch-unreachable]
   st->hctsiz_copy.b.xfersize = nrpackets * st->hcchar_copy.b.mps;
                                            ~~~~~~~~~~~~~~~~~^~~~

Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>