staging: comedi: serial2002: factor (*poll) busy wait out of tty_read()

Factor the (*poll) busy wait code out of tty_read() so the indent
level can be reduced and tty_read() is a bit cleaner.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>