serial: 8250: sparse warning of incorrect type

fixed a sparse warning in 8250_core.c :
	incorrect type in assignment (different address spaces)

the warning was because an unsigned char pointer was being assigned to
a pointer of unsigned char __iomem type .

Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>