ArmPlatformPkg: Remove double write in PL011

ArmPlatformPkg: Remove double write in PL011

The variable LineControl was initialised to zero, then updated in a
condition.  This change converts that to a write in each branch of the
condition, removing the Write/Read/Modify/Write sequence.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>
Signed-off-by: Evan Lloyd <evan.lloyd@arm.com>
Tested-by: Ryan Harkin <ryan.harkin@linaro.org>
Reviewed-by: Ryan Harkin <ryan.harkin@linaro.org>