PCI: endpoint: Fix missing destroy_workqueue()

PCI: endpoint: Fix missing destroy_workqueue()

Add the missing destroy_workqueue() before return from
pci_epf_test_init() in the error handling case and add
destroy_workqueue() in pci_epf_test_exit().

Link: https://lore.kernel.org/r/20210331084012.2091010-1-yangyingliang@huawei.com
Fixes: dbbde090d453c ("PCI: endpoint: functions: Add an EP function to test PCI")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>