staging: comedi: addi_apci_3501: Use insn->n in EEPROM insn_read handler

The `insn_read` handler for the EEPROM subdevice
(`apci3501_eeprom_insn_read()`) currently ignores `insn->n` (the number
of samples to be read) and assumes a single sample is to be read.  But
`insn->n` could be 0, meaning no samples should be read, in which case
`data[0]` ought not to be written.  (The comedi core at least ensures
that `data[0]` exists, but we should not rely on that.)

Following the usual Comedi guidelines and interpret `insn->n` as the
number of samples to be read, but only read the EEPROM location once and
make `insn->n` copies, as we don't expect the contents of the EEPROM
location to change between readings.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>