imx: serial: fix one bit field type

"have_rtscts" is assigned 1, while it is declared
int:1, two's complement, which can hold 0 and -1
only. The code works, as the upper bits are cut
off, and tests are done against 0 only.

Nonetheless, correctly declaring the bit field
as unsigned int:1 renders the code more robust.

Signed-off-by: Daniel Glöckner <dg@emlix.com>
Signed-off-by: Oskar Schirmer <os@emlix.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>