uart: native_posix: Check fcntl() return value

fcntl() may (very unlikely) fail when setting the new pseudoterminal
to non-blocking.
Let's check for this condition, handle it, and in the process
silence a coverity issue.

Fixes #14396
Coverity CID: 195872

Signed-off-by: Alberto Escolar Piedras <alpi@oticon.com>