Staging: irda: net: Use NOT operator instead of comparison to NULL

Fix issues find by checkpatch.pl.
CHECK: Comparison to NULL could be written "!dongles"
CHECK: Comparison to NULL could be written "!tasks"

Signed-off-by: Georgiana Chelu <georgiana.chelu93@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>