serial: icom: move header content to .c

There is no point keeping the header content separated. The header was
not even protected against double inclusion. So move the content to the
appropriate source file.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220421085808.24152-6-jslaby@suse.cz


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