e1000: Increase rx_buffer_size to 32b

Extend the size of the local rx_buffer_size variable to account for
larger buffer sizes possible on 82580, i350 chips.

From i350 datasheet, 6.2.10 Initialization Control 4 (LAN Base Address
+ Offset 0x13):
When 4 ports are enabled maximum buffer size is 36 KB. When 2 ports are
enabled maximum buffer size is 72 KB. When only a single port is
enabled maximum buffer size is 144 KB.

and 8.3:
The overall available internal buffer size in the I350 for all ports is
144 KB for receive buffers and 80 KB for transmit Buffers. Disabled
ports memory can be shared between active ports and sharing can be
asymmetric. The default buffer size for each port is loaded from the
EEPROM on initialization.

From the reporter:
But for I350 when only 2 ports are used PBA size can be set as 72KB
(see datasheet RXPbsize or e1000_rxpbs_adjust_82580 function in
e1000_82575.c). In this case calculating the rx_buffer_size overflows
as 0x0048 << 10 = 73728 or 0x12000 pushed into u16. It is then set as
0x2000 or 8192.

PR:		263896
Reported by:	hannula@gmail.com
Tested by:	hannula@gmail.com
Approved by:	markj
MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D35167

sys/dev/e1000/if_em.c

@@ -2477,7 +2477,7 @@ em_reset(if_ctx_t ctx)
 	struct e1000_softc *sc = iflib_get_softc(ctx);
 	struct ifnet *ifp = iflib_get_ifp(ctx);
 	struct e1000_hw *hw = &sc->hw;
-	u16 rx_buffer_size;
+	u32 rx_buffer_size;
 	u32 pba;
 
 	INIT_DEBUGOUT("em_reset: begin");