counter: fix an IS_ERR() vs NULL bug

There are 8 callers for devm_counter_alloc() and they all check for NULL
instead of error pointers.  I think NULL is the better thing to return
for allocation functions so update counter_alloc() and devm_counter_alloc()
to return NULL instead of error pointers.

Fixes: c18e2760

 ("counter: Provide alternative counter registration functions")
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20220111173243.GA2192@kili


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>