media: usb: cx231xx-417: fix a memory leak bug

In cx231xx_load_firmware(), 'p_buffer' is allocated through vmalloc() to
hold the firmware. However, after the usage, it is not deallocated, leading
to a memory leak bug.

Signed-off-by: Wenwen Wang <wenwen@cs.uga.edu>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>