driver core: fix driver_set_override() issue with empty strings

Python likes to send an empty string for some sysfs files, including the
driver_override field.  When commit 23d99baf9d72 ("PCI: Use
driver_set_override() instead of open-coding") moved the PCI core to use
the driver core function instead of hand-rolling their own handler, this
showed up as a regression from some userspace tools, like DPDK.

Fix this up by actually looking at the length of the string first
instead of trusting that userspace got it correct.

Fixes: 23d99baf9d72 ("PCI: Use driver_set_override() instead of open-coding")
Cc: Krzysztof Kozlowski <[email protected]>
Cc: Bjorn Helgaas <[email protected]>
Cc: "Rafael J. Wysocki" <[email protected]>
Cc: Andy Shevchenko <[email protected]>
Cc: stable <[email protected]>
Reported-by: Stephen Hemminger <[email protected]>
Tested-by: Huisong Li <[email protected]>
Reviewed-by: Stephen Hemminger <[email protected]>
Reviewed-by: Krzysztof Kozlowski <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

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