media: solo6x10: simplify the logic at solo_p2m_dma_desc()

The logic with gets a p2m_id is more complex than needed,
causing false positives with static analyzers:

	drivers/media/pci/solo6x10/solo6x10-p2m.c:81 solo_p2m_dma_desc() error: buffer overflow 'solo_dev->p2m_dev' 4 <= s32max

Make it simpler and use unsigned int.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>