qtnfmac: drop -D__CHECK_ENDIAN from cflags

Flag -D__CHECK_ENDIAN was wrong: it should be -D__CHECK_ENDIAN__ instead.
However now this flag is enabled by default, so it can be removed.

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