net: lan78xx: remove set but not used variable 'event'

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

drivers/net/usb/lan78xx.c:3995:6: warning:
 variable event set but not used [-Wunused-but-set-variable]

It is never used, so can be removed.

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