PCI: endpoint: pci-epf-test: Fix sleeping function being called from atomic context

When Root Complex (RC) triggers a Doorbell interrupt to Endpoint (EP), it
triggers the below warning in the EP:

BUG: sleeping function called from invalid context at kernel/locking/mutex.c:271
Call trace:
__might_resched+0x130/0x158
__might_sleep+0x70/0x88
mutex_lock+0x2c/0x80
pci_epc_get_msi+0x78/0xd8
pci_epf_test_raise_irq.isra.0+0x74/0x138
pci_epf_test_doorbell_handler+0x34/0x50

The BUG arises because the EP's pci_epf_test_doorbell_handler() which is
running in the hard IRQ context is making an indirect call to
pci_epc_get_msi(), which uses mutex inside.

To fix the issue, convert the hard IRQ handler to a threaded IRQ handler to
allow it to call functions that can sleep during bottom half execution.
Also, register the threaded IRQ handler with IRQF_ONESHOT to keep the
interrupt line disabled until the threaded IRQ handler completes execution.

Fixes: eff0c286aa91 ("PCI: endpoint: pci-epf-test: Add doorbell test support")
Signed-off-by: Bhanu Seshu Kumar Valluri <bhanuseshukumar@gmail.com>
[mani: reworded description a bit]
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Reviewed-by: Niklas Cassel <cassel@kernel.org>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Link: https://patch.msgid.link/20251014024109.42287-1-bhanuseshukumar@gmail.com