parisc: isa-eeprom - Fix loff_t usage

commit 6b4dbcd86a9d464057fcc7abe4d0574093071fcc upstream.

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>
Signed-off-by: Helge Deller <deller@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>