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'

Acked-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>