PCI: endpoint: Replace mdelay with usleep_range() in pci_epf_test_write()

pci_epf_test_write() is never called in atomic context.

The call chain ending up at pci_epf_test_write() is:
[1] pci_epf_test_write() <- pci_epf_test_cmd_handler()

pci_epf_test_cmd_handler() is set as a parameter of INIT_DELAYED_WORK()
in pci_epf_test_probe().
This function is not called in atomic context.

Despite never getting called from atomic context, pci_epf_test_write()
calls mdelay() to busy wait.

This is not necessary and can be replaced with usleep_range() to
avoid busy waiting.

Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
[lorenzo.pieralisi@arm.com: updated commit log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>