V4L/DVB (8531): mxl5007t: move i2c gate handling outside of mutex protected code blocks

There is no reason to protect the i2c gate handling within the mxl5007t
state mutex.

Thanks to Steven Toth for pointing this out.

Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>