staging: gdm724x: Correction of variable usage after applying ALIGN()

Fix regression introduced by commit <29ef8a53

>. After it writing
AT commands to /dev/GCT-ATM0 is unsuccessful (no echo, no response)
and dmesg show "gdmtty: invalid payload : 1 16 f011".

Before that commit value of dummy_cnt was only a padding size. After using
ALIGN() this value is increased by its first argument. So the following
usage of this variable needs correction.

Signed-off-by: Sławomir Demeszko <s.demeszko@wireless-instruments.com>
Cc: stable <stable@vger.kernel.org> # 3.14+
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>