serial: stm32: Move to more traditional formatting

Providing a new line, then an attribute on the same line as the
following closing bracket is odd.  Move to something more
transitional and in line with regular coding standards.

Change-Id: I08f7cd76ca2f522a8b5b6de2e5baf94e95b5195a
Signed-off-by: Lee Jones <lee.jones@linaro.org>