firmware: arm_ffa: Fix the comment style

clang produces the following warning:

    drivers/firmware/arm_ffa/driver.c:123: warning: expecting
    prototype for FF(). Prototype was for FFA_PAGE_SIZE() instead

    This comment starts with '/**', but isn't a kernel-doc comment.
    Refer Documentation/doc-guide/kernel-doc.rst

Fix the same by removing the kernel-doc style comment here.

Link: https://lore.kernel.org/r/20210622162202.3485866-1-sudeep.holla@arm.com


Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>