staging: sm750fb: Use memset_io instead of memset

This patch takes into account that cursor->vstart, crtc->vScreen and
share->pvMem are pointers to memory-mapped I/O and thus we should use memset_io
to make this explicit. In addition, some architectures require special treatment
of memory-mapped I/O so the previous code could actually break without this
change.

This fixes the following sparse warnings:-

drivers/staging/sm750fb/sm750.c:489:17: warning: incorrect type in argument 1 (different address spaces)
drivers/staging/sm750fb/sm750.c:490:17: warning: incorrect type in argument 1 (different address spaces)
drivers/staging/sm750fb/sm750.c:501:17: warning: incorrect type in argument 1 (different address spaces)
drivers/staging/sm750fb/sm750.c:502:17: warning: incorrect type in argument 1 (different address spaces)
drivers/staging/sm750fb/sm750.c:833:5: warning: incorrect type in argument 1 (different address spaces)
drivers/staging/sm750fb/sm750.c:1154:9: warning: incorrect type in argument 1 (different address spaces)

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