[media] em28xx: do not store eeprom content permanently

We currently reserve an array of 256 bytes for the eeprom content in the device
struct. For eeproms with 16 bit address width it might even be necessary to
increase the buffer size further.
Having such a big chunk of memory reserved even if the device has no eeprom and
keeping it after it has already been processed seems to be a waste of memory.
Change the code to allocate + free the eeprom memory dynamically.
This also makes it possible to handle different dataset sizes depending on what
is stored/found in the eeprom.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>