NetworkPkg/TlsAuthConfigDxe: Declare EFIAPI for the ChooseFile handler

NetworkPkg/TlsAuthConfigDxe: Declare EFIAPI for the ChooseFile handler

The ChooseFile handler, UpdateCAFromFile, has to be EFIAPI or gcc would
use the wrong ABI and cause the crash of the firmware. This commit also
removes the unnecessary type casting in ChooseFile.

Cc: Siyuan Fu <siyuan.fu@intel.com>
Cc: Jiaxin Wu <jiaxin.wu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Gary Lin <glin@suse.com>
Reviewed-by: Siyuan Fu <siyuan.fu@intel.com>
Reviewed-by: Jiaxin Wu <jiaxin.wu@intel.com>