m32r: serial: remove M32R_SIO_SHARE_IRQS

Remove an unused symbol M32R_SIO_SHARE_IRQS from drivers/serial/m32r_sio.h.

Signed-off-by: Hirokazu Takata <takata@linux-m32r.org>
Cc: "Robert P. J. Day" <rpjday@mindspring.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>