staging: sm750fb: Fix __iomem pointer types

This patch annotates pointers as referring to I/O mapped memory where they ought
to be, removes now unnecessary ugly casts, eliminates an incorrect deref on I/O
mapped memory by using iowrite16 instead, and updates the pointer arithmetic
accordingly to take into account that the pointers are now byte-sized. This
fixes the following sparse warnings:-

drivers/staging/sm750fb/sm750_cursor.c:113:19: warning: cast removes address space of expression
drivers/staging/sm750fb/sm750_cursor.c:204:19: warning: cast removes address space of expression

Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>