drm: bridge/dw_hdmi: avoid being recursive in N calculation

There's no need to be recursive when computing the N value for the ACR
packet - we can instead calculate the multiplier prior to our switch()
based lookup, and multiply the N value appropriately afterwards.

Tested-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>