taprio: validate TCA_TAPRIO_ATTR_FLAGS through policy instead of open-coding

As of now, the field TCA_TAPRIO_ATTR_FLAGS is being validated by manually
checking its value, using the function taprio_flags_valid().

With this patch, the field will be validated through the netlink policy
NLA_POLICY_MASK, where the mask is defined by TAPRIO_SUPPORTED_FLAGS.
The mutual exclusivity of the two flags TCA_TAPRIO_ATTR_FLAG_FULL_OFFLOAD
and TCA_TAPRIO_ATTR_FLAG_TXTIME_ASSIST is still checked manually.

Changes since RFC:
- fixed reversed xmas tree
- use NL_SET_ERR_MSG_MOD() for both invalid configuration

Changes since v1:
- Changed NL_SET_ERR_MSG_MOD to NL_SET_ERR_MSG_ATTR when wrong flags
  issued
- Changed __u32 to u32

Changes since v2:
- Added the missing parameter for NL_SET_ERR_MSG_ATTR (sorry again for
  the noise)

Signed-off-by: Alessandro Marcolini <alessandromarcolini99@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>