input: kbd_matrix: skip column with empty mask

The active key mask can be used for supporting multiple keyboard layouts
with a single firmware. A possible use case is to support keyboard with
or without a numpad, in which case an entire set of columns may be
missing. Add a check to detect this condition and skip scanning that
column entirely if no keys are defined in it.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>