[MTD] mtdchar: Return EINVAL for bad seeks instead of fixing up to valid byte

mtdchar return -EINVAL for seek prior to offset 0 or to beyond the last
byte in the device/partition, similar to various other seek methods,
instead of fixing up to first or last byte.

Signed-off-by: Todd Poynor <tpoynor@mvista.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>