staging: rtl8712: fix kmalloc parameters

After the commit 91d435fe replaced _malloc with kmalloc, smatch picked up a
couple of new warnings. This fixes warnings:

warn: struct type mismatch 'writePTM_parm vs setdig_parm'
warn: struct type mismatch 'writePTM_parm vs setra_parm'

The difference is u8 vs unsigned char.

Signed-off-by: Vitaly Osipov <vitaly.osipov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>