iio: buffer: Save a few cycles in 'iio_scan_mask_set()'

Use 'bitmap_alloc()' instead of 'bitmap_zalloc()' because the bitmap
is fully overridden by a 'bitmap_copy()' call just after its allocation.

While at it, fix the style of a NULL check.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/89d8a766eb971eda1ee362444a8711037bdb208c.1626261211.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>