ArmPkg/SemihostFs: Fix open file issues revealed by SCT

ArmPkg/SemihostFs: Fix open file issues revealed by SCT

Fix return codes in case of errors.
Fix translation from EFI open mode to semi-hosting open mode to avoid the
truncate to zero length of an already existing file opened in read/write
or read/write/create mode.

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@16241 6f19259b-4bc3-4df7-8a09-765794883524