staging: gpib: avoid unused const variables

staging: gpib: avoid unused const variables

Variables that are 'static const' but not used anywhere cause a warning
with "gcc -Wunused-const-variable", which we may want to enable by default
in the future.

The gpib code already has a mix of 'enum' and 'static const' variables
for named constants, so convert the ones that are causing problems to
enums as well, or move them closer to the only users where possible.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20241016111521.1143191-3-arnd@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>