mwifiex: change license text of Makefile and README from MARVELL to NXP

As of 6-DEC-2019, NXP has acquired Marvell’s Wireless business
unit. This change is to update the license text accordingly.

commit 932183aa ("mwifiex: change license text from MARVELL
to NXP") does this, but it left out two files.

Signed-off-by: James Cao <zheng.cao@nxp.com>
Signed-off-by: Cathy Luo <xiaohua.luo@nxp.com>
Signed-off-by: Ganapathi Bhat <ganapathi.bhat@nxp.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1606814307-32715-1-git-send-email-ganapathi.bhat@nxp.com