Staging: speakup: Use sizeof(*var) in kmalloc().

Staging: speakup: Use sizeof(*var) in kmalloc().

Modifying struct allocation in kmalloc() to match the
coding standards.

Checkpatch.pl CHECK: Prefer kmalloc(sizeof(*ldisc_data)...)
over kmalloc(sizeof(struct spk_ldisc_data)...)

Signed-off-by: Sam Muhammed <jane.pnx9@gmail.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Link: https://lore.kernel.org/r/19494bdab5709693126e0c0ee14b179a3b601207.1585046066.git.jane.pnx9@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>