net: fec_mxc: Change "error frame" message to debug level

As reported by Jonathan Gray:

"After the recent changes to add SimpleNetworkProtocol to efi_loader
when booting off mmc via an efi payload that doesn't use
SimpleNetworkProtocol U-Boot's fec_mxc driver will now display
various "error frame" messages.
....
MMC Device 1 not found
MMC Device 2 not found
MMC Device 3 not found
Scanning disks on sata...
Found 6 disks
reading efi/boot/bootarm.efi
67372 bytes read in 32 ms (2 MiB/s)
## Starting EFI application at 12000000 ...
>> OpenBSD/armv7 BOOTARM 1.0
error frame: 0x8f57ec40 0x00003d74
error frame: 0x8f57ec40 0x00007079
error frame: 0x8f57ec40 0x00006964
error frame: 0x8f57ec40 0x00006f6f
error frame: 0x8f57ec40 0x0000726f
error frame: 0x8f57ec40 0x00002074
error frame: 0x8f57ec40 0x00006f6f"

Heinrich Schuchardt explains:

"A receive FIFO overrun can be expected if network packages are not
processed.
With the network patches we check if a package is available quite often."

Move the "error frame" messages to debug level so that a clean output
log can be seen.

Reported-by: Jonathan Gray <jsg@jsg.id.au>
Suggested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>