i2c: Fix packet buffer storage size

The I2C binding packet buffer size was inadvertently reduced in
a3830d259a53 which could result in overflow.

Fixes: a3830d259a53 ("Fix test-i2c leak, and mctp_pktbuf storage alignment")
Change-Id: I99d11de014c0182378ebcdb631753010ccb5af44
Signed-off-by: Matt Johnston <matt@codeconstruct.com.au>
diff --git a/i2c-internal.h b/i2c-internal.h
index 95d245c..d2bc758 100644
--- a/i2c-internal.h
+++ b/i2c-internal.h
@@ -35,8 +35,10 @@
 
 	uint8_t own_addr;
 
-	uint8_t tx_storage[MCTP_PKTBUF_SIZE(I2C_BTU)] PKTBUF_STORAGE_ALIGN;
-	uint8_t rx_storage[MCTP_PKTBUF_SIZE(I2C_BTU)] PKTBUF_STORAGE_ALIGN;
+	uint8_t tx_storage[sizeof(struct mctp_i2c_hdr) +
+			   MCTP_PKTBUF_SIZE(I2C_BTU)] PKTBUF_STORAGE_ALIGN;
+	uint8_t rx_storage[sizeof(struct mctp_i2c_hdr) +
+			   MCTP_PKTBUF_SIZE(I2C_BTU)] PKTBUF_STORAGE_ALIGN;
 
 	mctp_i2c_tx_fn tx_fn;
 	void *tx_ctx;