tty: cpm_uart: use resource_size()

Use the resource_size function instead of manually calculating the
resource size.  This reduces the chance of introducing off-by-one errors.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Kumar Gala <galak@gate.crashing.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>