tests/periph_eeprom: remove useless casts

Casting to `const char *` is not necessary as input, the type are
already the right ones.

It could not be before as `result` was with the wrong type.

tests/periph_eeprom/main.c

@@ -226,7 +226,7 @@ static int cmd_test(int argc, char **argv)
 
     char result[4];
     ret = eeprom_read(0, (uint8_t *)result, 4);
-    assert(strncmp((const char *)result, (const char *)expected, 4) == 0);
+    assert(strncmp(result, expected, 4) == 0);
     assert(ret == 4);
 
     /* read/write at end of EEPROM */
@@ -234,7 +234,7 @@ static int cmd_test(int argc, char **argv)
     assert(ret == 4);
     memset(result, 0, 4);
     ret = eeprom_read(EEPROM_SIZE - 4, (uint8_t *)result, 4);
-    assert(strncmp((const char *)result, expected, 4) == 0);
+    assert(strncmp(result, expected, 4) == 0);
     assert(ret == 4);
 
     /* read/write single byte */
@@ -249,24 +249,24 @@ static int cmd_test(int argc, char **argv)
     eeprom_clear(0, 4);
     memset(result, 0, 4);
     ret = eeprom_read(0, (uint8_t *)result, 4);
-    assert(strncmp((const char *)result, "", 4) == 0);
+    assert(strncmp(result, "", 4) == 0);
     assert(ret == 4);
 
     eeprom_clear(EEPROM_SIZE - 4, 4);
     ret = eeprom_read(EEPROM_SIZE - 4, (uint8_t *)result, 4);
-    assert(strncmp((const char *)result, "", 4) == 0);
+    assert(strncmp(result, "", 4) == 0);
     assert(ret == 4);
 
     /* set some bytes */
     eeprom_set(0, 'A', 4);
     ret = eeprom_read(0, (uint8_t *)result, 4);
-    assert(strncmp((const char *)result, "AAAA", 4) == 0);
+    assert(strncmp(result, "AAAA", 4) == 0);
     assert(ret == 4);
 
     memset(result, 0, 4);
     eeprom_set(EEPROM_SIZE - 4, 'A', 4);
     ret = eeprom_read(EEPROM_SIZE - 4, (uint8_t *)result, 4);
-    assert(strncmp((const char *)result, "AAAA", 4) == 0);
+    assert(strncmp(result, "AAAA", 4) == 0);
     assert(ret == 4);
 
     puts("SUCCESS");