omap_hsmmc: improve interrupt synchronisation

The following changes were needed:
	- do not use in_interrupt() because it will not work
	with threaded interrupts

In addition, the following improvements were made:
	- ensure DMA is unmapped only after the final DMA interrupt
	- ensure a request is completed only after the final DMA interrupt
	- disable controller interrupts when a request is not in progress
	- remove the spin-lock protecting the start of a new request from
	an unexpected interrupt because the locking was complicated and
	a 'req_in_progress' flag suffices (since the spin-lock only defers
	the unexpected interrupts anyway)
	- instead use the spin-lock to protect the MMC interrupt handler
	from the DMA interrupt handler
	- remove the semaphore preventing DMA from being started while
	the previous DMA is still in progress - the other changes make that
	impossible, so it is now a BUG_ON condition
	- ensure the controller interrupt status is clear before exiting
	the interrrupt handler

In general, these changes make the code safer but do not fix any specific
bugs so backporting is not necessary.

Signed-off-by: Adrian Hunter <adrian.hunter@nokia.com>
Tested-by: Venkatraman S <svenkatr@ti.com>
Acked-by: Madhusudhan Chikkature <madhu.cr@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Cc: <linux-mmc@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>