net: usb: rtl8150: remove set but not used variable 'rx_stat'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/usb/rtl8150.c: In function 'read_bulk_callback':
drivers/net/usb/rtl8150.c:391:6: warning:
 variable 'rx_stat' set but not used [-Wunused-but-set-variable]

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>