mvebu: turris_omnia: Fix PEX vs SATA detection for board topology

The I2C reading in the PEX vs SATA detection code often fails on the
first try. Try three times, as the code for EEPROM reading does.

Signed-off-by: Marek Behun <marek.behun@nic.cz>
Signed-off-by: Stefan Roese <sr@denx.de>