powerpc/nvram_64: Check nvram_error_log_index in nvram_clear_error_log()

nvram_clear_error_log() calls ppc_md.nvram_write() even when
nvram_error_log_index is -1 (invalid). The nvram_write() function does
not check for a negative offset.

Check nvram_error_log_index as the other nvram log functions do.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: linuxppc-dev@ozlabs.org
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>