MdeModulePkg/FvSimpleFs: don't open DevicePath with BY_DRIVER mode

MdeModulePkg/FvSimpleFs: don't open DevicePath with BY_DRIVER mode

The device path protocol doesn't get closed at FvSimpleFilesystem
driver stop() when it's opened at start() with BY_DRIVER mode.

We change it to open DevicePath with GET_PROTOCOL mode as FV2 protocol with BY_DRIVER is enough.

Cc: Chao Zhang <chao.b.zhang@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Feng Tian <feng.tian@intel.com>
Reviewed-by: Chao Zhang <chao.b.zhang@intel.com>