serial: fsl_linflexuart: Be consistent with the name

For consistency reasons, spell the controller name as "LINFlexD" in
comments and documentation.

Signed-off-by: Stefan-Gabriel Mirea <stefan-gabriel.mirea@nxp.com>
Link: https://lore.kernel.org/r/1571230107-8493-4-git-send-email-stefan-gabriel.mirea@nxp.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>