EmbeddedPkg: Clarify the declaration of SerialPortSetControl()

EmbeddedPkg: Clarify the declaration of SerialPortSetControl()

Rework of PL011UartSetControl() as a consequence as well as update of
function header comment blocks.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ronald Cron <ronald.cron@arm.com>
Reviewed-By: Olivier Martin <olivier.martin@arm.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16251 6f19259b-4bc3-4df7-8a09-765794883524