power panic: simplify recovering SD filename

We can just read the whole EEPROM block since short filenames
are always null terminated. strcat_P will then apply the file extension
at the correct position.

Change in memory:
Flash: -24 bytes
SRAM: 0 bytes

Firmware/power_panic.cpp

@@ -438,11 +438,9 @@ void restore_file_from_sd()
         card.chdir(dir_name, false);
     }
 
-    for (uint8_t i = 0; i < 8; i++) {
+    // Recover DOS 8.3 filename without extension.
-        const char c = eeprom_read_byte((uint8_t*)EEPROM_FILENAME + i);
+    // Short filenames are always null terminated.
-        filename[i] = c;
+    eeprom_read_block(filename, (const char *)EEPROM_FILENAME, 8);
-        if (c == '\0') break; // Filename is shorter than 8 characters
-    }
 
     // Add null delimiter in case all 8 characters were not NULL
     filename[8] = '\0';