ipm: quark_se: fix dropped/repeating characters

The while loop on the status bit was unnecessary as we will get
a separate invocation of the ISR for every message. We also ensure
that the global status register bits for the channel being serviced
are cleared before exiting. Fixes an issue where some messages
would be dropped or repeated.

Some runtime checks that shouldn't happen in practice have been moved
to assertions.

Change-Id: Iedd28ae15522c7b9f59ef34fa9fa8b4e24e6be14
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>