misc: pci_endpoint_test: Fix to support > 10 pci-endpoint-test devices

Adding more than 10 pci-endpoint-test devices results in
"kobject_add_internal failed for pci-endpoint-test.1 with -EEXIST, don't
try to register things with the same name in the same directory". This
is because commit 2c156ac7 ("misc: Add host side PCI driver for PCI
test function device") limited the length of the "name" to 20 characters.
Change the length of the name to 24 in order to support upto 10000
pci-endpoint-test devices.

Fixes: 2c156ac7

 ("misc: Add host side PCI driver for PCI test function device")
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: stable@vger.kernel.org # v4.14+