ALSA: pcmtest: minor optimizations

Decrease the buffer filling overhead with conditional remainder
calculation in the 'inc_buf_pos' inline function.

Fix the driver to use already defined variables where it is possible
in 'check_buf_block_ni' and 'fill_block_pattern_n' functions.

Signed-off-by: Ivan Orlov <ivan.orlov0322@gmail.com>
Link: https://lore.kernel.org/r/20230713075953.13692-2-ivan.orlov0322@gmail.com


Signed-off-by: Takashi Iwai <tiwai@suse.de>