parisc: isa-eeprom - Fix loff_t usage

loff_t is a signed type. If userspace passes a negative ppos, the "count"
range check is weakened. "count"s bigger than HPEE_MAX_LENGTH will pass the check.
Also, if ppos is negative, the readb(eisa_eeprom_addr + *ppos) will poke in random
memory.

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Cc: stable@kernel.org
Signed-off-by: Helge Deller <deller@gmx.de>

drivers/parisc/eisa_eeprom.c

@@ -55,7 +55,7 @@ static ssize_t eisa_eeprom_read(struct file * file,
 	ssize_t ret;
 	int i;
 
-	if (*ppos >= HPEE_MAX_LENGTH)
+	if (*ppos < 0 || *ppos >= HPEE_MAX_LENGTH)
 		return 0;
 
 	count = *ppos + count < HPEE_MAX_LENGTH ? count : HPEE_MAX_LENGTH - *ppos;