Use masking and you don't need care about another bits:

    PCON |= 0x80; /* set SMOD*/
to clear SMOD you can use
    PCON &= (~0x80); /* clear SMOD*/
--------
You may also define SMOD bit in your own header file
such way:

#define SMOD_BIT    (0x80)
  or
#define SMOD_BIT    (1<<7)

and in 'C' file use it.

    PCON |= SMOD_BIT; /* set SMOD*/

    PCON &= (~SMOD_BIT); /* clear SMOD*/

Vaclav