atm: lanai: Remove in_interrupt() usage

lanai_shutdown_tx_vci() uses in_interrupt() to issue a warning message
if the function was used in context in which it is not safe to sleep.

The usage of in_interrupt() in driver code is deprecated as it can not always
detect all states where it is not allowed to sleep.

msleep() has debug code which will trigger a warning if used in bad
context.

Remove in_interrupt().

Cc: Chas Williams <3chas3@gmail.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>