staging: gpib: Remove atn_asserted global variable

staging: gpib: Remove atn_asserted global variable

atn_asserted was introduced to deal with an issue where certain models
of the Raspberry Pi would lose interrupts under heavy load. Using
a combination of edge and level interrupts the problem was resolved
so the work-around is no longer needed.

This patch removes the work-around.

Tested-by: Dave Penkler <dpenkler@gmail.com>
Signed-off-by: Dave Penkler <dpenkler@gmail.com>
Link: https://lore.kernel.org/r/20250418173434.2892-6-dpenkler@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>