1.正确 

PWMSUBM0 &= (byte)(PWMSUBM0 | 0xfc);

PWMSUBM0 &= (byte)(PWMSUBM0 | (byte)0xfc);

2.不能编译的

PWMSUBM0 &=(PWMSUBM0 | (byte)0xfc);

c#类似单片机的8bit或运算