xirc2ps_cs: fix register access when enabling FullDuplex

xirc2ps_cs: fix register access when enabling FullDuplex

The current code incorrectly passes (XIRCREG1_ECR | FullDuplex) as
the register address to GetByte(), instead of fetching the register
value and OR-ing it with FullDuplex. This results in an invalid
register access.

Fix it by reading XIRCREG1_ECR first, then or-ing with FullDuplex
before writing it back.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Alok Tiwari <alok.a.tiwari@oracle.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/20250827192645.658496-1-alok.a.tiwari@oracle.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>