usb: mtu3: fix dbginfo in qmu_tx_zlp_error_handler
Fixes gcc '-Wunused-but-set-variable' warning:
drivers/usb/mtu3/mtu3_qmu.c: In function 'qmu_tx_zlp_error_handler':
drivers/usb/mtu3/mtu3_qmu.c:385:22: warning:
variable 'req' set but not used [-Wunused-but-set-variable]
It seems dbginfo original intention is print 'req' other than 'mreq'