staging: dgap: tty.c: removes smatch warnings "redundant null check"

This patch removes these smatch warnings:
redundant null check on dgap_TmpWriteBuf calling kfree()
redundant null check on brd->SerialDriver->ttys calling kfree()
redundant null check on brd->PrintDriver->ttys calling kfree()

The code checked to see if these variables are null
before freeing. This check isn't needed.

Signed-off-by: Lidza Louina <lidza.louina@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>