serial: imx: rename variables to match the register names

Now the variable holding the value of register USR1 is called usr1
instead of sts which is more straight forward. The same is also done for
sts2 which is called usr2 now.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>