usb serial line speed limits

[Per olof Ljungmark]

I'm trying to get better speed from ubsa(4) to use a 3G modem att full 
speed. Editing ubsa.c up to 921600 is fine but if I go to 1228800 
compile fails:

cc -O2 -fno-strict-aliasing -pipe  -D_KERNEL -DKLD_MODULE -std=c99 
-nostdinc   -DHAVE_KERNEL_OPTION_HEADERS -include 
/usr/obj/usr/src/sys/HUNTER/opt_global.h -I. -I@ -I@/contrib/altq 
-finline-limit=8000 --param inline-unit-growth=100 --param 
large-function-growth=1000 -fno-common -g -I/usr/obj/usr/src/sys/HUNTER 
-mno-align-long-strings -mpreferred-stack-boundary=2  -mno-mmx 
-mno-3dnow -mno-sse -mno-sse2 -mno-sse3 -ffreestanding -Wall 
-Wredundant-decls -Wnested-externs -Wstrict-prototypes 
-Wmissing-prototypes -Wpointer-arith -Winline -Wcast-qual  -Wundef 
-Wno-pointer-sign -fformat-extensions -c 
/usr/src/sys/modules/ubsa/../../dev/usb/ubsa.c
/usr/src/sys/modules/ubsa/../../dev/usb/ubsa.c: In function 'ubsa_baudrate':
/usr/src/sys/modules/ubsa/../../dev/usb/ubsa.c:534: error: 'B1228800' 
undeclared (first use in this function)
/usr/src/sys/modules/ubsa/../../dev/usb/ubsa.c:534: error: (Each 
undeclared identifier is reported only once
/usr/src/sys/modules/ubsa/../../dev/usb/ubsa.c:534: error: for each 
function it appears in.)
*** Error code 1

Where can I dig out more info on how to proceed? Unfortunately my 
understanding of C and the rest is rather limited...

Thanks!