media: dvb-frontends/mxl5xx: cleanup and fix licensing boilerplates

mxl5xx.h doesn't carry any licensing boilerplate at all right now, so copy
the boilerplate over from the main driver file mxl5xx.c. Also, mxl5xx_defs
is missing a part of the licensing boilerplate text, so add it.

[mchehab@kernel.org: kept only the part of this patch that copied
 the license from mxl5xx.c into mxl5xx.h]
Cc: Ralph Metzler <rjkm@metzlerbros.de>
Cc: Manfred Voelkel <mvoelkel@DigitalDevices.de>
Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>