ARM: OMAP: Check STI channel value before registering console driver

Since ab7ad9fe425f03c9f83379509c44f98d155e1424 sti-console puts
crap into STI if serial console is configured as a preferred console.

This is due to uninitialized sti_console_channel (0 is a bad channel
value).

The fix is to check the STI channel value before registering the
console driver.

Signed-off-by: Mikko Ylinen <mikko.k.ylinen@nokia.com>
Signed-off-by: Juha Yrjola <juha.yrjola@solidboot.com>