virt: gunyah: Correct max_buf_size for a connection

Continuation messages can carry a maximum payload of
GH_RM_MAX_MSG_SIZE_BYTES. When initializing a connection buffer for
reply sequences, the hdr_size is 4 bytes larger than the
GH_RM_MAX_MSG_SIZE_BYTES: see the difference between
struct gh_rm_rpc_hdr and struct gh_rm_rpc_reply_hdr.

Fix the calculation.

Change-Id: If7a1124f581c9d1da8f7749d0296e064e6499cdf
Signed-off-by: Elliot Berman <quic_eberman@quicinc.com>