staging: gpib: Modernize gpib_interface_t initialization and make static

staging: gpib: Modernize gpib_interface_t initialization and make static

All interface drivers were using the old style initialization of
this struct

field : value;

This generated the followng sparse warning, for example:
agilent_82357a/agilent_82357a.c:1492:1: warning: obsolete struct initializer, use C99 syntax

Change the initialization to use the C99 syntax

.field = value;

This also resolves the checkpatch constraint of no indentation

These structs were also not declared as static, unnecessarily polluting
the symbol namespace and generating the following sparse warnings,
for example:

agilent_82357a/agilent_82357a.c:1465:18: warning: symbol 'agilent_82357a_gpib_interface' was not declared. Should it be static?

Declare them as static and remove any conflicting extern declarations
in the corresponding include files.

Signed-off-by: Dave Penkler <dpenkler@gmail.com>
Link: https://lore.kernel.org/r/20250111160514.26954-1-dpenkler@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>