nvmutil: reset errno on successful write

previously, a bad checksum would have caused a non-zero
exit, even if the other checksum was correct (observed
when using the swap command)

Signed-off-by: Leah Rowe <leah@libreboot.org>

util/nvmutil/nvmutil.c

@@ -269,5 +269,6 @@ writeGbeFile(void)
 		handle_endianness(p^x);
 		err_if(pwrite(fd, (uint8_t *) gbe[p^x], nf, p << 12) == -1);
 	}
+	errno = 0;
 	err_if(close(fd) == -1);
 }