staging: dgnc: replace unnecessary while() with if()

It doesn't need to use while loop for getting newrate,
because it always breaks out the end of while loop with
"break". So just replace while with if.

And the type of newrate is "unsigned int", this type
is never less than zero. If it can be set to negative value by
user application with ioctl(), it is not zero but it
can be a unexpected value for setting custom baudrate.

Also smatch says:
drivers/staging/dgnc/dgnc_tty.c:967 dgnc_set_custom_speed() warn:
 unsigned 'newrate' is never less than zero.
drivers/staging/dgnc/dgnc_tty.c:981 dgnc_set_custom_speed() info:
 ignoring unreachable code.

Signed-off-by: Daeseok Youn <daeseok.youn@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>