tty: serial: unify TX space reads under altera_jtaguart_tx_space()

TX space reads from the control register are performed in various forms
on 4 places in altera_jtaguart. Unify all those and do the read and
masking on a single place.

The new helper altera_jtaguart_tx_space() uses FIELD_GET(), so we can
drop ALTERA_JTAGUART_CONTROL_WSPACE_OFF now.

Cc: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Acked-by: Tobias Klauser <tklauser@distanz.ch>
Link: https://lore.kernel.org/r/20220927111819.18516-3-jslaby@suse.cz


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