Input: atmel_mxt_ts - improve T19 GPIO keys handling

 * The mapping of the GPIO numbers into the T19 status byte varies between
   different maXTouch chips. Some have up to 7 GPIOs. Allowing a keycode array
   of up to 8 items is simpler and more generic. So replace #define with
   configurable number of keys which also allows the removal of is_tp.
 * Rename platform data parameters to include "t19" to prevent confusion with
   T15 key array.
 * Probe aborts early on when pdata is NULL, so no need to check.
 * Move "int i" to beginning of function (mixed declarations and code)
 * Use API calls rather than __set_bit()
 * Remove unused dev variable.

Signed-off-by: Nick Dyer <nick.dyer@itdev.co.uk>
Acked-by: Yufeng Shen <miletus@chromium.org>
Reviewed-by: Henrik Rydberg <rydberg@euromail.se>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>