staging: gpib: Add return value to request_control

staging: gpib: Add return value to request_control

A number of drivers are unable to release control due to hardware or
software limitations. As request_system_control was defined as void,
no error could be signalled.

This patch changes the prototype of request_system_control to int and
adds the appropriate checking and returns. In the case that a board
cannot release control EINVAL is returned.  If a driver does not
implement request_system_control EPERM is returned.

Signed-off-by: Dave Penkler <dpenkler@gmail.com>
Link: https://lore.kernel.org/r/20250418133537.22491-1-dpenkler@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>