serial: sc16is7xx: reorder code to remove prototype declarations

Move/reorder some functions to remove sc16is7xx_ier_set() and
sc16is7xx_stop_tx() prototypes declarations.

No functional change.

sc16is7xx_ier_set() was introduced in
commit cc4c1d05 ("sc16is7xx: Properly resume TX after stop").

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>
Link: https://lore.kernel.org/r/20231221231823.2327894-16-hugo@hugovil.com


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