firmware: arm_ffa: Use the correct buffer size during RXTX_MAP

firmware: arm_ffa: Use the correct buffer size during RXTX_MAP

Don't use the discovered buffer size from an FFA_FEATURES call directly
since we can run on a system that has the PAGE_SIZE larger than the
returned size which makes the alloc_pages_exact for the buffer to be
rounded up.

Fixes: 61824feae5c0 ("firmware: arm_ffa: Fetch the Rx/Tx buffer size using ffa_features()")
Signed-off-by: Sebastian Ene <sebastianene@google.com>
Link: https://patch.msgid.link/20260402113939.930221-1-sebastianene@google.com
Signed-off-by: Sudeep Holla <sudeep.holla@kernel.org>