QCanBusFrame: Apply Q_ENUM to the FrameError enum again

d772275d278a333486d87e8046444bc21afe6cd0 removed it when porting Q_FLAGS
to Q_FLAG. While this shouldn't matter for the QMetaObject, it matters
for other meta-type interactions with the enum itself (notably
QMetaEnum::fromType, which no longer worked). Fix this by adding Q_ENUM
once more.

Pick-to: 6.11 6.10
Change-Id: I8031be8066ae14db7525a8b9f75c017e671f03fc
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
Reviewed-by: André Hartmann <aha_1980@gmx.de>
Reviewed-by: Ahmad Samir <a.samirh78@gmail.com>