tty: serial: fsl_lpuart: rename register variables more specifically

There are many fuzzy register variables in the lpuart driver, such as
temp, tmp, val, reg. Let's give these register variables more specific
names.

Signed-off-by: Sherry Sun <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: SherrySun5