[media] pms: Fix a bad usage of the stack

As warned by smatch:
	drivers/media/parport/pms.c:632:21: warning: Variable length array is used.

The pms driver is doing something really bad: it is using the
stack to read data into a buffer whose size is given by the
user by the read() syscall. Replace it by a dynamically allocated
buffer.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>