Enhance period_index accuracy, particularly just before buffer rewind, by
making use of DMA interrupt status flags in addition to simply counting up
interrupts.

Created against linux-2.6.31-rc5.

Tested on Amstrad Delta.
Signed-off-by: NJanusz Krzysztofik <jkrzyszt@tis.icnet.pl>
Acked-by: NJarkko Nikula <jhnikula@gmail.com>
Signed-off-by: NMark Brown <broonie@opensource.wolfsonmicro.com>