uart.c: Improve error handling
uart.c: Improve error handling

Changed syslog messages and some error returns.

Signed-off-by: Martin G Lane-Smith <2b7a6c561ce522715a4c9d5c508221e008ac3093@gmail.com>
Signed-off-by: Brendan Le Foll <3e0e8c4069934e8b47d99704927a85c66c0ceb33@intel.com>

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