TTY: isdn, make some functions readable

Huh, this was a mess.

* Remove the 5 indent levels by just returning from isdn_tty_try_read
  when the conditions there are not satisfied.
* Use 'continue' in loop in isdn_tty_readmodem to save 2 levels.
* Chain 'if's in isdn_tty_modem_escape.
* Use 'continue' in loop in isdn_tty_carrier_timeout to save 1 level.

Better to look at this patch with -w -b.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Karsten Keil <isdn@linux-pingi.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>