serial: Reorder serial_assign()

Reorder serial_assign() function to get rid of the extra level of
indentation. Also, adjust the return value to be -EINVAL instead of
positive one to be more consistent.

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