misc: ibmvmc: Repair ill-named function argument descriptions

misc: ibmvmc: Repair ill-named function argument descriptions

Formatting is correct and descriptions are all present, but some
of the argument names are not properly represented in the
documentation.

Fixes the following W=1 build warnings:

 drivers/misc/ibmvmc.c:780: warning: Function parameter or member 'msg_len' not described in 'ibmvmc_send_msg'
 drivers/misc/ibmvmc.c:780: warning: Excess function parameter 'msg_length' description in 'ibmvmc_send_msg'
 drivers/misc/ibmvmc.c:1041: warning: Function parameter or member 'buffer' not described in 'ibmvmc_write'
 drivers/misc/ibmvmc.c:1041: warning: Excess function parameter 'buf' description in 'ibmvmc_write'
 drivers/misc/ibmvmc.c:1360: warning: Function parameter or member 'file' not described in 'ibmvmc_ioctl'
 drivers/misc/ibmvmc.c:1360: warning: Excess function parameter 'session' description in 'ibmvmc_ioctl'

Cc: Steven Royer <seroyer@linux.ibm.com>
Cc: Adam Reznechek <adreznec@linux.vnet.ibm.com>
Cc: "Bryant G. Ly" <bryantly@linux.vnet.ibm.com>
Cc: Dave Engebretsen <engebret@us.ibm.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20200701083118.45744-30-lee.jones@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>