MdeModulePkg/UefiBootManagerLib: Remove redundant type cast

MdeModulePkg/UefiBootManagerLib: Remove redundant type cast

The type of return value for function EfiBootManagerFindLoadOption() is
INTN. When checking its return value, it is unnecessary to type cast -1 to
type UINTN.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Sunny Wang <sunnywang@hpe.com>