thunderbolt: Call tb_eeprom_get_drom_offset() from tb_eeprom_read_n()

We are going to re-use tb_drom_read() for USB4 DROM reading as well.
USB4 has separate router operations for this which does not need the
drom_offset. Therefore we move call to tb_eeprom_get_drom_offset() into
tb_eeprom_read_n() where it is needed.

While there change return -ENOSYS to -ENODEV because the former is only
supposed to be used with system calls (invalid syscall nr).

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Link: https://lore.kernel.org/r/20191217123345.31850-3-mika.westerberg@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>