net: wilc1000: clean up resource in error path of init mon interface

The wilc_wfi_init_mon_int() forgets to clean up resource when
register_netdevice() failed. Add the missed call to fix it.
And the return value of netdev_priv can't be NULL, so remove
the unnecessary error handling.

Fixes: 58871300 ("staging: wilc1000: avoid the use of 'wilc_wfi_mon' static variable")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Huang Guobin <huangguobin4@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200917123019.206382-1-huangguobin4@huawei.com