[MTD] CORE mtdchar.c: fix off-by-one error in lseek()

Allow lseek(mtdchar_fd, 0, SEEK_END) to succeed, which currently fails
with EINVAL.

lseek(fd, 0, SEEK_END) should result into the same fileposition as
lseek(fd, 0, SEEK_SET) + read(fd, buf, length(fd))

Furthermore, lseek(fd, 0, SEEK_CUR) should return the current file position,
which in case of an encountered EOF should not result in EINVAL

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>