cfag12864b fix

This one-liner patch fixes a bug in drivers/auxdisplay/cfag12864b.c

At cfag12864b_init(), the driver tries to kalloc some memory in the
variable cfag12864b_cache.

Then, as usual, it checks if the call failed. However, it checks
cfag12864b_buffer instead.

This patch changes the "cfag12864b_buffer" to "cfag12864b_cache" so the
correct variable is checked.

Signed-off-by: Miguel Ojeda <maxextreme@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>