serial: Fix _serial_puts using \n\r instead of \r\n

A string like "test\n" would be broken up into the following sequence of
prints by _serial_puts:

	puts("test\n")
	putc('\r')

Although functionally this is the same as \r\n, it is not the standard
sequence and caused tests to fail. Fix this by excluding the '\n' from
the initial print. The above string will now be broken up like

	puts("test")
	puts("\r\n")

Since we may now need to call ops->puts twice (with the associated retry
logic), break that part of the function off into a helper.

Fixes: 7a763471 ("serial: dm: Add support for puts")
Signed-off-by: Sean Anderson <sean.anderson@seco.com>