staging: gpib: Add missing mutex unlock in agilent usb driver

staging: gpib: Add missing mutex unlock in agilent usb driver

When no matching product id was found in the attach function the driver
returned without unlocking the agilent_82357a_hotplug_lock mutex.

Add the unlock call.

This was detected by smatch:
smatch warnings:
drivers/staging/gpib/agilent_82357a/agilent_82357a.c:1381 agilent_82357a_attach() warn: inconsistent returns 'global &agilent_82357a_hotplug_lock'.

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/r/202412210143.WJhYzXfD-lkp@intel.com/
Fixes: 4c41fe886a56 ("staging: gpib: Add Agilent/Keysight 82357x USB GPIB driver")
Signed-off-by: Dave Penkler <dpenkler@gmail.com>
Link: https://lore.kernel.org/r/20250111161457.27556-1-dpenkler@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>