qtnfmac: modify qtnf_map_bar not to return NULL

NULL is not a special type of success here but a error pointer.
So it makes sense to check against NULL in qtnf_map_bar
and return error code.

Signed-off-by: Sergey Matyukevich <sergey.matyukevich.os@quantenna.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>