[pcbios] Do not switch to real mode to check for timer interrupt

The currticks() function is called at least once per TCP packet, and
so is performance-critical.  Switching to real mode just to allow the
timer interrupt to fire is expensive when running inside a virtual
machine, and imposes a significant performance cost.

Fix by enabling interrupts without switching to real mode.  This
results in an approximately 100% increase in download speed when
running under KVM.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/interface/pcbios/bios_timer.c

 	uint8_t midnight;
 
 	/* Re-enable interrupts so that the timer interrupt can occur */
-	__asm__ __volatile__ ( REAL_CODE ( "sti\n\t"
-					   "nop\n\t"
-					   "nop\n\t"
-					   "cli\n\t" ) : : );
+	__asm__ __volatile__ ( "sti\n\t"
+			       "nop\n\t"
+			       "nop\n\t"
+			       "cli\n\t" );
 
 	get_real ( ticks, BDA_SEG, 0x006c );
 	get_real ( midnight, BDA_SEG, 0x0070 );