firmware: arm_ffa: Switch to using ffa_mem_desc_offset() accessor

firmware: arm_ffa: Switch to using ffa_mem_desc_offset() accessor

In preparation to add support to the new memory transaction descriptor,
the ep_mem_access member needs to be removed and hence even the macro
COMPOSITE_OFFSET(). Let us switch to using the new ffa_mem_desc_offset()
accessor in ffa_setup_and_transmit().

This will enable adding the support for new format transparently without
any changes here again.

Link: https://lore.kernel.org/r/20231005-ffa_v1-1_notif-v4-15-cddd3237809c@arm.com
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>