hw/timer/imx_epit: Simplify and fix imx_epit implementation

hw/timer/imx_epit: Simplify and fix imx_epit implementation

When imx_epit.c was last refactored, a common usecase (comparison
register zero) broke.  This patch fixes that, and simplifies the code
yet more.  It also fixes a major thinko in the reset path --- the
wrong bits in the control register were being cleared.

Signed-off-by: Peter Chubb <peter.chubb@nicta.com.au>
Reviewed-by: Jean-Christophe DUBOIS <jcd@tribudubois.net>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>