Input: stmfts - fix setting ABS_MT_POSITION_* maximum size

The commit 78bcac7b ("Input: add support for the STMicroelectronics
FingerTip touchscreen) used the 'touchscreen_parse_properties()' helper
function in order to get the value of common properties.

But, commit 78bcac7b didn't set the capability of ABS_MT_POSITION_*
before calling touchscreen_parse_properties(). In result, the max_x and
max_y of 'struct touchscreen_properties' were not set.

Fixes: 78bcac7b ("Input: add support for the STMicroelectronics FingerTip touchscreen")
Cc: stable@vger.kernel.org
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>