dmaengine: acpi: Check for errors from acpi_register_gsi() separately

While IRQ test agaist the returned variable in practice is a good enough
there is still a room for theoretical mistake in case the vIRQ of the
device contains the same error code that acpi_register_gsi() may return.
Due to this, check for error code separately from matching the vIRQs.

Besides that, append documentation to tell why acpi_gsi_to_irq() can't
be used and we call acpi_register_gsi() instead.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20210802175532.54311-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>