iio: light: stk3310: Use sizeof() for regmap_bulk_read/write count parameter

iio: light: stk3310: Use sizeof() for regmap_bulk_read/write count parameter

Convert the hardcoded count parameter to sizeof(buf) for all
regmap_bulk_write() and regmap_bulk_read() calls in this driver
to improve code maintainability. For details, see [1].

Link: https://lore.kernel.org/all/20260428192213.7c5c80e5@jic23-huawei/
Signed-off-by: Miao Li <limiao@kylinos.cn>
Reviewed-by: Joshua Crofts <joshua.crofts1@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>