[media] s5h1420: Don't use dynamic static allocation

Dynamic static allocation is evil, as Kernel stack is too low, and
compilation complains about it on some archs:
	drivers/media/dvb-frontends/s5h1420.c:851:1: warning: 's5h1420_tuner_i2c_tuner_xfer' uses dynamic stack allocation [enabled by default]
Instead, let's enforce a limit for the buffer.
In the specific case of this frontend, only ttpci uses it. The maximum
number of messages there is two, on I2C read operations. As the logic
can add an extra operation, change the size to 3.

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>
Reviewed-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>