hal: various fixes for MMAP NO IRQ mode

- Fix NULL reference in error report
- Adjust period count according to requested buffer size in
create_mmap_buffer()
- Exit standby in create_mmap_buffer()
- Do not call pcm_prepare() before pcm_start() as this is reduncdant.

Bug: 35260844
Test: CTS test_aaudio.cpp
Change-Id: Id7a171e26023fb9fc7a10eed47b1eb96be37c75d
Signed-off-by: Phil Burk <philburk@google.com>