将变量 nSectorSize 修改为常量，以提高代码可读性

[Liewzheng]

为两个变量新增 const 修饰，以防在 Mac M1 Apple Silicon 下编译失败。

g++ -DHAVE_CONFIG_H -I. -I./cfg  -Wall -Werror -Wextra -Wreturn-type -fno-strict-aliasing -D_FILE_OFFSET_BITS=64 -D_LARGE_FILE -I/opt/local/include/libusb-1.0   -g -O2 -MT main.o -MD -MP -MF .deps/main.Tpo -c -o main.o main.cpp
main.cpp:2563:12: error: variable length arrays in C++ are a Clang extension [-Werror,-Wvla-cxx-extension]
 2563 |         BYTE pBuf[nSectorSize * DEFAULT_RW_LBA];
      |                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
main.cpp:2563:12: note: read of non-const variable 'nSectorSize' is not allowed in a constant expression
main.cpp:2562:6: note: declared here
 2562 |         int nSectorSize = 512;
      |             ^
main.cpp:2935:12: error: variable length arrays in C++ are a Clang extension [-Werror,-Wvla-cxx-extension]
 2935 |         BYTE pBuf[nSectorSize * DEFAULT_RW_LBA];
      |                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
main.cpp:2935:12: note: read of non-const variable 'nSectorSize' is not allowed in a constant expression
main.cpp:2934:6: note: declared here
 2934 |         int nSectorSize = 512;
      |             ^
2 errors generated.
make[1]: *** [main.o] Error 1
make: *** [all-recursive] Error 1