qcacld-3.0: Rename pMac in lim_send_management_frames.c

Per the Linux coding style "mixed-case names are frowned upon" and
"so-called Hungarian notation [...] is brain damaged." One identifier
used extensively throughout the driver that violates both of these
tenants is "pMac." In order to be compliant with the Linux coding
style there is a campaign to rename all instances of this identifier.

For this change rename all instances of "pMac" to "mac" in:
lim_send_management_frames.c

Change-Id: I738a19602eca080f8fdd8d302cc9c17878a0a929
CRs-Fixed: 2355444