[int13] Do not zero %edx when jumping to a boot sector

Commit 73eb3f1 ("[int13] Zero all possible registers when jumping to a
boot sector") introduced a regression preventing the SAN-booting of
boot sectors which rely upon %dl containing the correct drive number
(such as most CD-ROM boot sectors).

Fix by not zeroing %edx.

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

src/arch/i386/image/bootsector.c

 					   "xorl %%eax, %%eax\n\t"
 					   "xorl %%ebx, %%ebx\n\t"
 					   "xorl %%ecx, %%ecx\n\t"
-					   "xorl %%edx, %%edx\n\t"
+					   /* %edx contains drive number */
 					   "xorl %%esi, %%esi\n\t"
 					   "xorl %%edi, %%edi\n\t"
 					   "xorl %%ebp, %%ebp\n\t"