MIPS: kernel: Rename read/write_c0_ecc to read/writec0_errctl

CP0 register 26 is used as ECC register for legacy cores, but newer
cores (MIPS32/MIPS64) use it as an ErrCtl register. Since the kernel only
uses CP0 26 as ErrCtl register rename the access functions to the more
fitting name.

Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Reviewed-by: Maciej W. Rozycki <macro@orcam.me.uk>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>