Properly spell out the whole structure member names when an initialized
varible is instantiated from the struct serial_driver. In case the
structure definition for struct serial_driver undergoes reordering,
there will be no impact on variables defined based on this structure.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Marek Vasut <marek.vasut@gmail.com>
Cc: Tom Rini <trini@ti.com>
Cc: Anatolij Gustschin <agust@denx.de>
Cc: Stefan Roese <sr@denx.de>
Cc: Mike Frysinger <vapier@gentoo.org>
Cc: C Nauman <cnauman@diagraph.com>
Cc: Minkyu Kang <mk7.kang@samsung.com>
Cc: Michal Simek <monstr@monstr.eu>