[ip6] fix conversion of enum value to uint8_t (#4522)
[ip6] fix conversion of enum value to uint8_t (#4522)

This commit updates how the `otMessagePriority` enumeration variable
is updated in `otMessageSettings` structure. This addresses an issue
where the enum variable was being treated as an `uint8_t` which may
not necessarily be correct (since complier can choose any integral
type which covers all the enumerator values as the underlying storage
for an unscoped enumeration type).
librasungirl/openthread,bukepo/openthread,openthread/openthread,jwhui/openthread,srickardti/openthread,jwhui/openthread,lanyuwen/openthread,librasungirl/openthread,openthread/openthread,abtink/openthread,lanyuwen/openthread,srickardti/openthread,chshu/openthread,srickardti/openthread,chshu/openthread,chshu/openthread,lanyuwen/openthread,openthread/openthread,chshu/openthread,bukepo/openthread,abtink/openthread,bukepo/openthread,chshu/openthread,lanyuwen/openthread,lanyuwen/openthread,lanyuwen/openthread,jwhui/openthread,jwhui/openthread,chshu/openthread,bukepo/openthread,librasungirl/openthread,librasungirl/openthread,abtink/openthread,openthread/openthread,abtink/openthread,srickardti/openthread