fix(iec): disable IRQs before releasing CLK to send a byte

iec_tx_byte and iec_tx_command released CLK and only then disabled IRQs. An
IRQ in between took about 500 cycles, longer than a listener's EOI wait and
acknowledgement, so the talker waited for an acknowledgement that had come and
gone, and a LOAD over the bus hung after its file name. Which byte an IRQ hit
depended on where the build placed the code.

src/kernal/iec/iec_tx_command.s

@@ -32,7 +32,10 @@ iec_tx_command:
 	; (can be busy processing something), we have to wait till they are all
 	; ready (or bored with DOS attack...)
 	; Release back CLK, keep DATA released - to ask devices for status.
+	; From here on IRQs are disabled, as in iec_tx_byte: a listener takes a
+	; talker that waits too long after the release for EOI
 
+	sei
 	jsr iec_release_clk_data
 
 	; Common part of iec_txbyte and iec_tx_common - waits for devices