add 2.6 ms delay to piface_lcd_clear() and piface_lcd_home()

 *  2.6 ms delay after LCD_CLEARDISPLAY
 *
 *  Execution time of the "Clear display" command is not specified
 *  in the HITACHI date sheet HD44780U, page 24. (Probably a misprint)
 *  It was measured and found to be 1.6 to 2.4 ms +- 0.2 ms

 *  2.6 ms delay after LCD_RETURNHOME
 *
 *  Execution time of the "Return home" command is specified as 1.52 ms
 *  in the HITACHI date sheet HD44780U, page 24. (Probably meant for
 *  "Clear display")
 *  It was measured and found to be less than 0.8 ms and is probably
 *  37 us like all other commands.  To be safe the delay was added here
 *  also as it hardly influences performance.

change SEQOP_ON to SEQOP_OFF in piface_open_noinit()

These values set bit 5 of the IOCON register in the MCP23S17 I/O Expander. Setting this bit to 0 (SEQOP_ON) enables address pointer increments in the MCP23S17. I do not think this is ever used (especially with BANK_OFF). Every command to the MCP23S17 sets a new address. The auto-incrementing of text byte transfers in the HD44780U is handled somewhere else.