MdedulePkg: AtaAtapiPassThru: Remove polling on PxCMD.FR flag setting

MdedulePkg: AtaAtapiPassThru: Remove polling on PxCMD.FR flag setting

It is enough to set PxCMD.FRE bit, which cause HBA to post received FISes
into the FIS receive area. According to AHCI Specification, only polling on
PxCMD.FRE to be cleared is necessary, when it is needeed to stop FIS engine
(eg. in order to change PxCMD.FB address).

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jan Dabros <jsd@semihalf.com>
Signed-off-by: Marcin Wojtas <mw@semihalf.com>
Reviewed-by: Feng Tian <feng.tian@intel.com>