serial: Reorder get_current()

Reorder the get_current() function to make it a bit more readable.
The code does not grow and there is minor change in the code logic,
where dev != NULL is now checked in any case.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Marek Vasut <marek.vasut@gmail.com>
Cc: Tom Rini <trini@ti.com>