xtensa simdisk: Fix proc_create_data() conversion fallout

arch/xtensa/platforms/iss/simdisk.c: In function 'proc_read_simdisk':
arch/xtensa/platforms/iss/simdisk.c:220:12: warning: initialization discards 'const' qualifier from pointer target type [enabled by default]
arch/xtensa/platforms/iss/simdisk.c: In function 'proc_write_simdisk':
arch/xtensa/platforms/iss/simdisk.c:241:38: error: 'buffer' undeclared (first use in this function)
arch/xtensa/platforms/iss/simdisk.c:241:38: note: each undeclared identifier is reported only once for each function it appears in

Introduced by commit a69755b1

 ("xtensa
simdisk: switch to proc_create_data()")

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>