net: phy: nxp-c45-tja11xx: read PHY the speed from hardware

Read PHY speed from hardware instead of assuming 100Mbps by default.
The TJA1103 works only at 100Mbps, but the driver will support more PHYs.

Signed-off-by: "Radu Pirea (NXP OSS)" <radu-nicolae.pirea@oss.nxp.com>