USB: serial: pl2303: account for deficits of clones

USB: serial: pl2303: account for deficits of clones

There are apparently incomplete clones of the HXD type chip in use.
Those return -EPIPE on GET_LINE_REQUEST and BREAK_REQUEST. Avoid
flooding the kernel log with those errors. Detect them during startup
and then use the line_settings cache instead of GET_LINE_REQUEST. Signal
missing break support via -ENOTTY.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
[ johan: fix macro prefix, drop oom error message ]
Signed-off-by: Johan Hovold <johan@kernel.org>