net: ti: icssg-prueth: Fix start counter for ft1 filter

The start counter for FT1 filter is wrongly set to 0 in the driver.
FT1 is used for source address violation (SAV) check and source address
starts at Byte 6 not Byte 0. Fix this by changing start counter to
ETH_ALEN in icssg_ft1_set_mac_addr().

Fixes: e9b4ece7

 ("net: ti: icssg-prueth: Add Firmware config and classification APIs.")
Signed-off-by: MD Danish Anwar <danishanwar@ti.com>
Link: https://lore.kernel.org/r/20240527063015.263748-1-danishanwar@ti.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>