pl011: incorrect IBRD_MASK and FBRD_MASK

Currently the masks are defined as

#define IBRD_MASK 0x3f
#define FBRD_MASK 0xffff

https://github.com/qemu/qemu/blob/a3fb4e93a3a7cf2be355c41cd550bef856f5ffe4/hw/char/pl011.c#L93

But it actually has to be vice versa: IBRD is 16 bit long and FBRD is 6 bit long.

https://developer.arm.com/documentation/ddi0183/latest/programmers-model/register-descriptions/fractional-baud-rate-register--uartfbrd

https://developer.arm.com/documentation/ddi0183/latest/programmers-model/register-descriptions/integer-baud-rate-register--uartibrd