Input: ads7846 - optimize order of calculating Rt in ads7846_rx()

Alter the if expression for calculating Rt. The old implementation would
run unnecessary code when the ADS7843 device was used.

The patch also fixes the code style to kernel standard.

Signed-off-by: Hans-Christian Egtvedt <hans-christian.egtvedt@atmel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>